spi: spi-synquacer: Fixed build on architectures missing readsl/writesl series

kbuild test reported that alpha and some of the architectures
are missing readsl/writesl series.
Use more portable ioread32_rep()/iowrite32_rep() series.

Fixes: b0823ee3 ("spi: Add spi driver for Socionext SynQuacer platform")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>