microblaze: Add support for little-endian Microblaze

Microblaze little-endian toolchain exports __MICROBLAZEEL__
which is used in the kernel to identify little/big endian.

The most of the changes are in loading values from DTB which
is always big endian.

Little endian platforms are based on new AXI bus which has
impact to early uartlite initialization.

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