EFI: update documentation for arm64

The arm64 EFI boot support added a new 'dtb' value to the configuration file.
Update the documentation to describe this and how the coniguration file is not
used when GRUB loads the modules. Updates 'ucode' description to indicate that
it is x86 only.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

docs/misc/efi.markdown

@@ -1,8 +1,19 @@
-Building xen.efi requires gcc 4.5.x or above (4.6.x or newer recommended, as
-4.5.x was probably never really tested for this purpose) and binutils 2.22 or
-newer. Additionally, the binutils build must be configured to include support
-for the x86_64-pep emulation (i.e. `--enable-targets=x86_64-pep` or an option
-of equivalent effect should be passed to the configure script).
+For x86, building xen.efi requires gcc 4.5.x or above (4.6.x or newer
+recommended, as 4.5.x was probably never really tested for this purpose) and
+binutils 2.22 or newer.  Additionally, the binutils build must be configured to
+include support for the x86_64-pep emulation (i.e.
+`--enable-targets=x86_64-pep` or an option of equivalent effect should be
+passed to the configure script).
+
+For arm64, the PE/COFF header is open-coded in assembly, so no toolchain
+support for PE/COFF is required.  Also, the PE/COFF header co-exists with the
+normal Image format, so a single binary may be booted as an Image file or as an
+EFI application.  When booted as an EFI application, Xen requires a
+configuration file as described below unless a bootloader, such as GRUB, has
+loaded the modules and describes them in the device tree provided to Xen.  If a
+bootloader provides a device tree containing modules then any configuration
+files are ignored, and the bootloader is responsible for populating all
+relevant device tree nodes.
 
 Once built, `make install-xen` will place the resulting binary directly into
 the EFI boot partition, provided `EFI_VENDOR` is set in the environment (and
@@ -73,7 +84,14 @@ Specifies an XSM module to load.
 
 ###`ucode=<filename>`
 
-Specifies a CPU microcode blob to load.
+Specifies a CPU microcode blob to load. (x86 only)
+
+###`dtb=<filename>`
+
+Specifies a device tree file to load.  The platform firmware may provide a
+DTB in an EFI configuration table, so this field is optional in that
+case. A dtb specified in the configuration file will override a device tree
+provided in the EFI configuration table. (ARM only)
 
 ###`chain=<filename>`