spi: mediatek: add no_need_unprepare support

spi: mediatek: add no_need_unprepare support

This patch add no_need_unprepare support for spi, if spi src clk is
MAIN PLL, it can keep the clk_prepare and will not cause low power
issue. So we no need do clk_prepare/clk_unprepare in runtime pm,
and it will get better performance, because clk_prepare has called
mutex lock.
In the same way,
clk_get_rate also has called mutex lock, so we moved it to spi_probe.

Signed-off-by: Mason Zhang <Mason.Zhang@mediatek.com>
Link: https://lore.kernel.org/r/20210629100814.21402-1-mason.zhang@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>