board: tbs2910: Gate clock when switching async clock muxes

board: tbs2910: Gate clock when switching async clock muxes

According to the i.MX6Q Reference Manual, clocks must be gated when
switching input clocks of async clock muxes. So use clock gates. Avoid
ldb_di0_ipu clock, because there is no clock gate for this signal.

There have never been any complaints about problems with the old code,
but the new approach is in line with the recommendations in the manual.

Signed-off-by: Soeren Moch <smoch@web.de>