spi/tegra-sflash: Factor runtime PM out into transfer prepare/unprepare

Currently the tegra sflash driver acquires a runtime PM reference for the
duration of each transfer. This may result in the IP being powered down
between transfers which would be at best wasteful. Instead it is better
to do this in the callbacks that are generated before and after starting
a series of transfers, keeping the IP powered throughout.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>