Enable parent rate propagation for clk_gate2 to allow the
clock consumers to adjust their rates.
One effect of this is that the i.MX6 NAND controller now can adjust
its rate. It already called a clk_set_rate(rate, 96000000), but this
had no effect, so the clock stayed at reset default 24MHz resulting
in a rather slow timing. This became a problem when commit
"1daa3bc mtd: nand_mxs: Setup timing" introduced EDO timing mode for
faster NAND chips. EDO mode can only work properly for cycle times
< 30ns (at least that's specified in the ONFI spec). 1daa3bc resulted
in sporadic NAND read errors on some boards.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Sam Ravnborg <sam@ravnborg.org>