spi: mxs: Allow other chip selects to work

MXS SSP controller may have up to three chip selects per port: SS0, SS1 and SS2.

Currently only SS0 is supported in the mxs_spi driver.

Allow all the three chip select to work by selecting the desired one
in bits 20 and 21 of the HW_SSP_CTRL0 register.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>