mx6sxsabresd: support qspi flash bigger than 16MB

mx6sxsabresd revb board uses 32MB qspi flash, reva board uses 16MB qspi
flash. Currently, the default supported platform is revb board.

If want to configure for reva board, just define CONFIG_MX6SX_SABRESD_REVA
in mx6sxsabresd.h to support reva qspi flashes whose size is 16MB.

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