powerpc/mpc8536ds: Add eSPI support for MPC8536DS

1. The SD_DATA[4:7] signals are shared with the SPI chip selects on 8536DS,
   so don't set MPC85xx_PMUXCR_SD_DATA that config eSDHC data bus-width
   to 4-bit and enable SPI signals.
2. Add eSPI controller and SPI-FLASH definition.

Signed-off-by: Xie Xiaobo <r63061@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>