mxs: Use XFER_COUNT field in HW_SSP_CTRL0 register in PIO mode

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
mx23: Add SSP registers map

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
mx23evk: Add initial board support

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
mxs: Add force PIO mode support for MMC

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
marek