spi: sirf: add support for USP-based SPI

USP is a general purpose serial port in SiRFSoC, which can work as SPI.
the most data flow of USP and pure SPI is same with main differences
in registers layout.
this patch moves registers layout to private data, and use flags to
differentiate other minor differences between prima2-spi, prima2-usp
and atlas7-usp for hardware configuration.

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