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: NMark 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: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/memory.h

@@ -55,8 +55,9 @@
 #define VMEMMAP_SIZE (UL(1) << (VA_BITS - PAGE_SHIFT - 1 + STRUCT_PAGE_MAX_SHIFT))
 
 /*
- * PAGE_OFFSET - the virtual address of the start of the kernel image (top
+ * PAGE_OFFSET - the virtual address of the start of the linear map (top
  *		 (VA_BITS - 1))
+ * KIMAGE_VADDR - the virtual address of the start of the kernel image
  * VA_BITS - the maximum number of bits for virtual addresses.
  * VA_START - the first kernel virtual address.
  * TASK_SIZE - the maximum size of a user space task.