APPLIED: README: document the requirements for CONFIG_SYS_HZ

CONFIG_SYS_HZ must be 1000, and get_timer() must therefore return ms.
Document this.

README text provided by Tom Rini.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
---
v2: Reworded the README text.
ARM: rpi_b: attempt to turn on power to SD controller

This can't be done yet: The firmware gives an error reponse to
set_power_state. See:
https://github.com/raspberrypi/firmware/issues/106

However, this isn't needed in practice, since the boot firmware must
have turned on the power itself, since it loaded U-Boot from the SD
card.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
ARM: bcm2835: fix get_timer() to return ms

Apparently, CONFIG_SYS_HZ must be 1000. Change this, and fix the timer
driver to conform to this.

Have the timer implementation export a custom API get_timer_us() for use
by the BCM2835 MMC API, which needs us resolution for a HW workaround.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
---
v2: Export custom API get_timer_us() to allow the MMC driver to maintain
    its current workaround implementation.