mmc: clk_scaling: add sysfs for avoiding scale down in write

Change Ia7a578434d7db8983493788ab2d7f9383d703169 introduced
a degradation in performance.
That change tried to fix a problem for clock scaling
during write requests.
During write request the invalid_state is always set due to
CMD13 that is sent after each write.
In the attempt to fix that issue the change above added the
ability to scale down although invalid_state was set.
Thus, creating a favor for downscale over upscale.
Some customers prefer performance over power.
This change introduces a sysfs entry that will allow the
driver to retain its old behavior which had better performance.

It can be controlled from new sysfs entry:
"scale_down_in_low_wr_load" found under
/sys/class/mmc_host/mmcX/clk_scaling/
The default value for it is "0". In case customers
want to gain performance over power they should set
it to "1".

Change-Id: I8fb1f430b6287526567521382486c2b11952aa73
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>