microblaze: Fix DTB passing from bootloader

Little endian system needs to check OF_DT_HEADER
but it is swapped because it is in big-endian.
Microblaze LE provides lwr instruction which loads
magic number in BIG endian format which can be compared.

There is used the fact that if you write 0x1 as word
and load it as byte then you get for big-endian zero
and 1 for little-endian.

Signed-off-by: Michal Simek <monstr@monstr.eu>