[POWERPC] ucc_geth: use rx-clock-name and tx-clock-name device tree properties

Updates the ucc_geth device driver to check the new rx-clock-name and
tx-clock-name properties first.  If present, it uses the new function
qe_clock_source() to obtain the clock source.  Otherwise, it checks the
deprecated rx-clock and tx-clock properties.

Update the device trees for 832x, 836x, and 8568 to contain the new property
names only.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>