at91: drop boot_params and PLAT_PHYS_OFFSET

select ARM_PATCH_PHYS_VIRT
as with ARM_PATCH_PHYS_VIRT you can patch boot_params at runtime or any recent
bootloader will provide a valid atags pointer in r2

as point out by Russell on AT91 we never use XIP
so se do not need PLAT_PHYS_OFFSET

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Patrice Vilchez <patrice.vilchez@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>