USB: phy-msm-usb: Add support for voting for nominal frequencies

Currently USB driver has support for voting for bus frequencies
passed through device tree bus parameters. With this, it is not
possible to vote for maximum nominal bus frequenices from USB to
DDR path. Hence add support for voting nominal values which helps
in mitigating thermal issue.

CRs-Fixed: 722888
Change-Id: I5697cd5dc7f3bd1bee448fe3aada57cca482fdeb
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>