mtd: mxs_nand: don't get the gpmi_apbh_dma clock

This clock name is not present in any U-boot and Linux kernel 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>