colibri_vf: use same NAND clock as Linux uses

colibri_vf: use same NAND clock as Linux uses

Currently a divider of 6 has been used, leading to following NAND
Flash Controller (NFC) clocks:
VF61: 27.7 MHz (166.7MHz bus clock)
VF50: 22 MHz (132MHz bus clock)

The NAND Flash Memory used on VF50 allows to use clock speed of
up to 33MHz, while the Flash Memory of VF61 allows 50MHz. We can
use the same divider of 4 on both modules to configure the maximal
possible clock speeds:
VF61: 41.7 MHz
VF50: 33 MHz

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>