mtd: mxs_nand: get the clock with the right name

Rename the gpmi_apb_bch clock name to gpmi_bch_apb, as you can find in
the device tree.

Fixes: commit a5969128 ("MXS_NAND: Add clock support for iMX8")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>