CONFIG_REGULATOR_FIXED_VOLTAGE

AM335x 或 BBB 的 kernel 有用到，不然會有下列錯誤

[    0.984058] omap_hsmmc 48060000.mmc: unable to get vmmc regulator -517
[    0.991028] platform 48060000.mmc: Driver omap_hsmmc requests probe deferral
[    0.998717] omap_hsmmc 481d8000.mmc: unable to get vmmc regulator -517
[    1.005611] platform 481d8000.mmc: Driver omap_hsmmc requests probe deferral
[    1.019195] omap_rtc 44e3e000.rtc: setting system clock to 2000-01-01 00:00:0
0 UTC (946684800)                                                              
[    1.031031] Waiting for root device /dev/mmcblk0p2...                       
[    1.037015] omap_hsmmc 48060000.mmc: unable to get vmmc regulator -517
[    1.044030] platform 48060000.mmc: Driver omap_hsmmc requests probe deferral
[    1.051768] omap_hsmmc 481d8000.mmc: unable to get vmmc regulator -517
[    1.058698] platform 481d8000.mmc: Driver omap_hsmmc requests probe deferral