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>