driver: clk: Add support for clocks on Armada 37xx

driver: clk: Add support for clocks on Armada 37xx

The drivers are based on Linux driver by Gregory Clement.

The TBG clocks support only the .get_rate method.
  - since setting rate is not supported, the driver computes the rates
    when probing and so subsequent calls to the .get_rate method do not
    read the corresponding registers again

The peripheral clocks support methods .get_rate, .enable and .disable.

  - the .set_parent method theoretically could be supported on some clocks
    (the parent would have to be one of the TBG clocks)

  - the .set_rate method would have to try all the divider values to find
    the best approximation of a given rate, and it doesn't seem like
    this should be needed in U-Boot, therefore not implemented

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>