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>
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v2: Export custom API get_timer_us() to allow the MMC driver to maintain
    its current workaround implementation.