frequency in the at91_pmc_clock_init rather than passing it in. Allow
for frequencies >= 21MHz by rounding to the nearest 500Hz (Idea from
Ian Lapore whose company uses a similar arrangement in their product).
at91_pmc_clock_init() is now nearly independent of the rest of the pmc
driver (which means we may be able to call it much earlier in boot
soon to eliminate the master clock config file requirement for printf
to work during early boot and also eliminate some interdependencies
with the device ordering which requires pmc to be the first device
added).
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