spi: fsl_qspi: fix compile warning for 64-bit platform

This patch fixes the following compile warning:
drivers/spi/fsl_qspi.c: In function 'fsl_qspi_probe':
drivers/spi/fsl_qspi.c:937:15:
  warning: cast to pointer from integer of different size
					 [-Wint-to-pointer-cast]
  priv->regs = (struct fsl_qspi_regs *)plat->reg_base;
               ^
Just make the cast explicit.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <york.sun@nxp.com>