Add support for enabling the USB on the Raspberry Pi boards when it hasn't
been done by U-Boot. This allows the USB to work when we load the kernel
directly.

No dma sync is performed after these operations as the data we read/write
is not used by the cpu after the calls to the maimbox driver.

Differential Revision: https://reviews.freebsd.org/D1940
Reviewed by: imp, Michal Meloun (meloun AT miracle.cz)
MFC after: 1 Week
Sponsored by: ABT Systems Ltd