arm64: Update the MAINTAINERS entry

Add a backup maintainer and include Documentation/arm64/.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

MAINTAINERS

@@ -1239,9 +1239,11 @@ F:	arch/arm/mach-pxa/include/mach/z2.h
 
 ARM64 PORT (AARCH64 ARCHITECTURE)
 M:	Catalin Marinas <catalin.marinas@arm.com>
+M:	Will Deacon <will.deacon@arm.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 F:	arch/arm64/
+F:	Documentation/arm64/
 
 ASC7621 HARDWARE MONITOR DRIVER
 M:	George Joseph <george.joseph@fairview5.com>