[ARM] pxa: correct SSCR0_SCR to support multiple SoCs

The previous definitions of SSCR0_SCR and SSCR0_SerClkDiv() prevented
them being used simultaneously when supporting multiple PXA SoCs, esp.
in drivers/spi/pxa2xx_spi.c, make them correct.

The change from SSCR0_SerClkDiv(2) to SSCR0_SCR(2), will make the result
a little bit different in pxa2xx_spi_probe(), however, since that's only
used as a default initialization value, it's acceptable.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>