spi: sirf: add missed spi mode_bits that SiRFSoC hardware supports

Missing this will cause some user cases fail when they want to change spi
transfer mode.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@linaro.org>