imx:mx6sxsabresd add qspi support

imx:mx6sxsabresd add qspi support

Configure the pad setting and enable qspi clock to support qspi
flashes access.

Add QSPI related macro in configuration header file.

Note:
mx6sxsabresd Revb board, 32M flash is used, but in header file,
CONFIG_SPI_FLASH_BAR is not defined, and we still use SZ_16M.
The LUT initialization qspi_set_lut function uses 32BIT addr,
however CONFIG_SPI_FLASH_BAR  and 24BIT addr should be used to
access bigger than 16MB size flash, and BRRD/BRWR should also
be supported. Future patches will fix this.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>