[POWERPC] qe: add function qe_clock_source()

Add function qe_clock_source() which takes a string containing the name of a
QE clock source (as is typically found in device trees) and returns the
matching enum qe_clock value.

Update booting-without-of.txt to indicate that the UCC properties rx-clock
and tx-clock are deprecated and replaced with rx-clock-name and tx-clock-name,
which use strings instead of numbers to indicate QE clock sources.

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