of/fdt: Populate phys_initrd_start/phys_initrd_size from FDT

Now that we have central and global variables holding the physical
address and size of the initrd, we can have
early_init_dt_check_for_initrd() populate
phys_initrd_start/phys_initrd_size for us.

This allows us to remove a chunk of code from arch/arm/mm/init.c
introduced with commit 65939301 ("arm: set initrd_start/initrd_end
for fdt scan").

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>