MIPS: Replace SIBYTE_1956_WAR by CONFIG_SB1_PASS_2_WORKAROUNDS

SB1250 uart bug is related to PASS 2 workarounds. Use config
CONFIG_SB1_PASS_2_WORKAROUNDS directly and get rid of SIBYTE_1956_WAR.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>