MIPS: TXx9: Add prom_getenv

Add prom_getenv() which can be used for YAMON.  This assumes other
firmware should pass NULL for fw_arg2.

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