microblaze: usable uart16550 for big endian systems

As a result of the commit 68332608 the uart16550 driver
is broken for Microblaze big endian systems, because of
the missing 3 byte offset. Other than as described, not
all U-Boot BSP will treat properly the 3 byte offset.

This why prefer to mask out the 3 byte offset in general
and setup correct _REG_SIZE value depending on edianess.

Signed-off-by: Stephan Linz <linz@li-pro.net>
Tested-by: Michal Simek <monstr@monstr.eu>