usb: phy: msm add regulator dependency

On linux-4.4 and linux-4.9 we get a warning about an array that is
never initialized when CONFIG_REGULATOR is disabled:

drivers/usb/phy/phy-msm-usb.c: In function 'msm_otg_probe':
drivers/usb/phy/phy-msm-usb.c:1911:14: error: 'regs[0].consumer' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  motg->vddcx = regs[0].consumer;
  ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-msm-usb.c:1912:14: error: 'regs[1].consumer' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  motg->v3p3  = regs[1].consumer;
  ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-msm-usb.c:1913:14: error: 'regs[2].consumer' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  motg->v1p8  = regs[2].consumer;

This adds a Kconfig dependency for it. In newer kernels, the driver no
longer exists, so this is only needed for stable kernels.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>