mmc: core: Bypass clock scaling while accessing RPMB partition

According to eMMC specification, only commands of classes 0, 2, 4
are admitted while accessing RPMB partition. Still usage of any
other command other than CMD0, CMD6, CMD8, CMD12, CMD13, CMD15,
CMD18, CMD23, CMD25 is illegal.

If the MMC clock scaling algorithm decides to switch the clocks
while accessing RPMB partition it might need to send tuning
command (CMD21) which is illegal.

Since RPMB accesses are short and doesn't depend on throughput
bypass clock scaling while the current partion mode is RPMB.
The clock scaling statistics still take into account the duration
of access and hence able to respond quickly on the transfers made
after partition switch.

Change-Id: I422f2e6acb33ab97105944e3f7f90c3afb37ef47
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>