i2c: mv64xxx: handle probe deferral for the clock

If a clock is registered by a platform driver and not by the
OF_CLK_DECLARE() mechanism, it might show up after the first attempt
to probe i2c-mv64xxx. In order to solve this, we need to handle
-EPROBE_PREFER as a special return value of devm_clk_get(), and return
the same error code from probe().

This gives us three situations:

 - There is no reference to a clock in the DT. In this case,
   devm_clk_get() returns an error that is not -EPROBE_DEFER
   (something like -ENODEV), and we continue the probing without
   enabling the clock.

 - There is a reference to the clock in the DT, and the clock is
   ready. devm_clk_get() returns a valid reference to the clock, and
   we prepare/enable it.

 - There is a reference to the clock in the DT, but the clock is not
   ready. devm_clk_get() returns -EPROBE_DEFER, and we exit from
   probe() with the same error code so that probe() is tried again
   later.

This is needed for Marvell Armada 7K/8K, where the clock driver is a
platform driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>