xen/arm64: macros: Introduce an assembly macro to alias x30

The return address of a function is always stored in x30. For convenience,
introduce a register alias so "lr" can be used in assembly.

This is defined in asm-arm/arm64/macros.h to allow all assembly files
to use it.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm64/entry.S

@@ -6,11 +6,6 @@
 #include <asm/smccc.h>
 #include <public/xen.h>
 
-/*
- * Register aliases.
- */
-lr      .req    x30             /* link register */
-
 /*
  * Stack pushing/popping (register pairs only). Equivalent to store decrement
  * before, load increment after.

xen/include/asm-arm/arm64/macros.h

@@ -21,5 +21,10 @@
     ldr     \dst, [\dst, \tmp]
     .endm
 
+/*
+ * Register aliases.
+ */
+lr      .req    x30             /* link register */
+
 #endif /* __ASM_ARM_ARM64_MACROS_H */