arm64/entry: deduplicate SW PAN entry/exit routines

Factor the 12 copies of the SW PAN entry and exit code into callable
subroutines, and use alternatives patching to either emit a 'bl'
instruction to call them, or a NOP if h/w PAN is found to be available
at runtime.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20200721083315.4816-1-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>