m4: add ext_5v platform device, needed for OTG

Gets rid of a ton of these:
<3>[    0.090767] msm-gpio-regulator msm-gpio-regulator.97: Failed to find supply ext_5v
<3>[    0.090889] gpio_vreg_probe: ext_otg_sw: regulator_register failed, rc=-517.
<6>[    0.090920] platform msm-gpio-regulator.97: Driver msm-gpio-regulator requests probe deferral