[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>

include/asm-powerpc/qe.h

@@ -84,6 +84,7 @@ extern int par_io_data_set(u8 port, u8 pin, u8 val);
 
 /* QE internal API */
 int qe_issue_cmd(u32 cmd, u32 device, u8 mcn_protocol, u32 cmd_input);
+enum qe_clock qe_clock_source(const char *source);
 int qe_setbrg(enum qe_clock brg, unsigned int rate, unsigned int multiplier);
 int qe_get_snum(void);
 void qe_put_snum(u8 snum);