ufs: Add support for clock gating

The UFS controller clocks can be gated after certain period of
inactivity, which is typically less than runtime suspend timeout.
In addition to clocks the link will also be put into Hibern8 mode
to save more power.

The clock gating can be turned on by enabling the capability
UFSHCD_CAP_CLK_GATING. To enable entering into Hibern8 mode as part of
clock gating, set the capability UFSHCD_CAP_HIBERN8_WITH_CLK_GATING.

The tracing events for clock gating can be enabled through debugfs as:
echo 1 > /sys/kernel/debug/tracing/events/ufs/ufshcd_clk_gating/enable
cat /sys/kernel/debug/tracing/trace_pipe

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>