ARM: P2V: eliminate head.S use of PHYS_OFFSET for !XIP_KERNEL

head.S makes use of PHYS_OFFSET.  When it becomes a variable, the
assembler won't understand this.  Compute PHYS_OFFSET by the following
method.  This code is linked at its virtual address, but run at before
the MMU is enabled, so at his physical address.

1:	.long	.
	.long	PAGE_OFFSET

	adr	r0, 1b		@ r0 = physical ','
	ldmia	r0, {r1, r2}	@ r1 = virtual '.', r2 = PAGE_OFFSET
	sub	r1, r0, r1	@ r1 = physical-virtual
	add	r2, r2, r1	@ r2 = PAGE_OFFSET + physical-virtual
				@    := PHYS_OFFSET.

Switch XIP users of PHYS_OFFSET to use PLAT_PHYS_OFFSET - we can't
use this method for XIP kernels as the code doesn't execute in RAM.

Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>