spi: fsl: add missing __iomem annotation

The field mspi->reg_base is annotated as an __iomem pointer. Good.

However, this field is often assigned to a temporary variable:
before being used. For example:
	struct fsl_spi_reg *reg_base = mspi->reg_base;

But this variable is missing the __iomem annotation.
So, add the missing __iomem and make sparse & the bot happier.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Link: https://lore.kernel.org/r/20200622162611.83694-1-luc.vanoostenryck@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>