arm64: update stale PAGE_OFFSET comment

Commit ab893fb9 ("arm64: introduce KIMAGE_VADDR as the virtual
base of the kernel region") logically split KIMAGE_VADDR from
PAGE_OFFSET, and since commit f9040773

 ("arm64: move kernel
image to base of vmalloc area") the two have been distinct values.

Unfortunately, neither commit updated the comment above these
definitions, which now erroneously states that PAGE_OFFSET is the start
of the kernel image rather than the start of the linear mapping.

This patch fixes said comment, and introduces an explanation of
KIMAGE_VADDR.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>