spi: s3c64xx: extend driver to make full use of runtime PM autosuspend

Extend the driver to make full use of runtime PM autosuspend.
Before only the SPI core was instructed to use autosuspend
by setting master->auto_runtime_pm. Nevertheless due to the missing
pm_runtime_use_autosuspend call autosuspend wasn't active.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>