nios2: add 10m50 devboard support

Add 10m50 devboard support. It is based on the Golden Hardware
Reference Design (GHRD), available at,

http://rocketboards.org/foswiki/view/Documentation/


AlteraMAX1010M50RevCDevelopmentKitLinuxSetup

Though we supported only one nios2-generic board in the past. Now,
with the removal of the nios2-generic board dir, adding new nios2
boards to u-boot is easier than before. It should be helpful to
add those boards supported in Linux mainline. There are only two
such nios2 boards, the 3c120 devboard and 10m50 devboard. The
nios2-generic is actually 3c120, and should restore the name. The
10m50 is this one.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>