arm64: head: remove __PHYS_OFFSET

It's very easy to confuse __PHYS_OFFSET and PHYS_OFFSET. To clarify
things, let's remove __PHYS_OFFSET and use KERNEL_START directly, with
comments to show that we're using physical address, as we do for other
objects.

At the same time, update the comment regarding the kernel entry address
to mention __pa(KERNEL_START) rather than __pa(PAGE_OFFSET).

There should be no functional change as a result of this patch.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20220629041207.1670133-1-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>