USB: phy: msm: Add support for overriding SSPHY deemphasis setting

TX deemphasis value can be different from platform to platform.
This value has to be set to suitable value to pass USB3 complaince
test. Hence add support for overriding this deemphasis value from
device tree.

CRs-Fixed: 536912
Change-Id: Ic3c2213fc0d090407057525115eae1351ecc4668
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>