MIPS: TXx9: Fix spi-baseclk value

TXx9 SPI bit rate is calculated by:
	fBR = fSPI / 2 / (n + 1)
	(fSPI is SPI master clock freq, i.e. imbusclk freq.)
So use imbus_clk / 2 as a spi-baseclk.

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