Before reparenting the ENFC_SEL mux we have to make sure the GPMI_IO clk
is disabled as recommended in the reference manual. Otherwise glitches
may occur on the GPMI clk which result in a later NAND failure:

MXS: Timeout resetting block via register 0x01806000
mxs_nand 1806000.gpmi-nand@1806000.of: probe failed: Connection timed out

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>