USB: host: ehci-msm2: Add debugfs entry to access phy registers

Add a debugfs entry to write and read ULPI PHY registers, using the
below commands:
Debugfs location: <debugfs>/msm_ehci_host.1/

echo 0xADDR > phy_reg_addr

To write value to the register 0xADDR:
echo 0xVALUE > phy_reg_data

To read from the register 0xADDR:
cat phy_reg_data

CRs-Fixed: 491755
Change-Id: I3529fc257d107cec8b0525d6b344a1e51153f053
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>