ARM: 7941/2: Fix incorrect FDT initrd parameter override

Commit 65939301 (arm: set initrd_start/initrd_end for fdt scan)
caused the FDT initrd_start and initrd_end to override the
phys_initrd_start and phys_initrd_size set by the initrd= kernel
parameter.  With this patch initrd_start and initrd_end will be
overridden if phys_initrd_start and phys_initrd_size are set by the
kernel initrd= parameter.

Fixes: 65939301

 (arm: set initrd_start/initrd_end for fdt scan)

Signed-off-by: Ben Peddell <klightspeed@killerwolves.net>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>