Convert to USB_HIST.

From Takahiro HAYASHI with tweaks from me.
add some more bit definitions
Use SDn_CLK_REG instead of MMC_CLKCR to setup bus clock; gives a nearly 2x boost in performance in HS mode. Also, make sure to setup PLL6 before probing clocks.
Introduce RPI_HWCURSOR and default to off. The existing code causes
problems with vchiq at the moment.