Move AMD SB800 early clock setup.

Move AMD SB800 early clock setup.

Move the AMD SB800 early clock setup code that is needed for early
serial port operation from mainboard/romstage.c to sb800/bootblock.c.
This prevents code duplication and simplifies porting.

Change-Id: I615cfec96c9f202d9c154dc6674ec7cbcf4090c3
Signed-off-by: Scott Duplichan <scott@notabs.org>
Signed-off-by: Marc Jones <marcj303@gmail.com>
Reviewed-on: http://review.coreboot.org/96
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>