mtd: Fix endianness issues from device tree

This patch adds the appropriate conversions to correct the endianness
issues in the MTD driver whenever it accesses the device tree (which is
always big endian).

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>