ARM: rpi_b: power on SDHCI and USB HW modules

Send RPC commands to the VideoCore to turn on the SDHCI and USB modules.
For SDHCI this isn't needed in practice, since the firmware already
turned on the power in order to load U-Boot. However, it's best to be
explicit. For USB, this is necessary, since the module isn't powered
otherwise. This will allow the kernel USB driver to work.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>