docs/Makefile: Add ppc and riscv to DOC_ARCHES

Not having ppc and riscv included in DOC_ARCHES causes "multiple
definitions of ..." message on documentation build, similar to the
example shown below:

include/public/arch-ppc.h:91: multiple definitions of Typedef
vcpu_guest_core_regs_t: include/public/arch-arm.h:300
include/public/arch-ppc.h:91: multiple definitions of Typedef
vcpu_guest_core_regs_t: include/public/arch-ppc.h:85

It can also make the generated html documentation link to the header
files of another architecture. This is additionally a problem as it can
randomly make the documentation build non-reproducible.

Signed-off-by: Maximilian Engelhardt <maxi@daemonizer.de>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/Makefile

@@ -5,7 +5,7 @@ include $(XEN_ROOT)/Config.mk
 VERSION		:= $(shell $(MAKE) -C $(XEN_ROOT)/xen --no-print-directory xenversion)
 DATE		:= $(shell date +%Y-%m-%d)
 
-DOC_ARCHES      := arm x86_32 x86_64
+DOC_ARCHES      := arm ppc riscv x86_32 x86_64
 MAN_SECTIONS    := 1 5 7 8
 
 # Documentation sources to build