arm64: update PAC description for kernel

Remove the paragraph which has nothing to do with the kernel and
add PAC description related to kernel.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Link: https://lore.kernel.org/r/20211201034014.20048-1-Kuan-Ying.Lee@mediatek.com


Signed-off-by: Will Deacon <will@kernel.org>