arm: bcm283x: serial: Move ofdata reading to probe() method

arm: bcm283x: serial: Move ofdata reading to probe() method

We cannot rely on a parent bus that needs to be probed, until we know that
it is probed. That means that code in the ofdata_to_platdata() method
cannot rely on the parent bus being probed.

Move the ofdata code in the two serial drivers into a probe() method.

This fixes serial output on rpi_3b_32b with the following config.txt
options:

   enable_uart=1
   gpu_freq=250

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>