sparc: Support for GRLIB APBUART serial port

This patch adds support for the APBUART serial port from Aeroflex
Gaisler's IP library GRLIB. It is currently used in all LEON3 designs
(SPARC V8) but can be used on other platforms as well (which support OF).

Signed-off-by: Kristoffer Glembo <kristoffer@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>