[MIPS] TXx9: Make single kernel can support multiple boards

Make single kernel can be used on RBTX4927/37/38.  Also make
some SoC-specific code independent from board-specific code.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>