Documentation: Add ARM64 to kernel-parameters.rst

Add ARM64 to the legend of architectures.  It's already used in several
places in kernel-parameters.txt.
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/admin-guide/kernel-parameters.rst

@@ -88,6 +88,7 @@ parameter is applicable::
 	APIC	APIC support is enabled.
 	APM	Advanced Power Management support is enabled.
 	ARM	ARM architecture is enabled.
+	ARM64	ARM64 architecture is enabled.
 	AX25	Appropriate AX.25 support is enabled.
 	CLK	Common clock infrastructure is enabled.
 	CMA	Contiguous Memory Area support is enabled.