spi/pxa2xx: fix runtime PM enabling order

In commit 7dd62787 (spi/pxa2xx: Convert
to core runtime PM) master->auto_runtime_pm was set to true.

In this case pm_runtime_enable() must be called *before*
spi_register_master(), otherwise the kernel hangs with this error
message:

  spi_master spi0: Failed to power device: -13

A similar fix, but for spi/hspi, was applied in
268d7643.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Mark Brown <broonie@linaro.org>