colibri-imx6ull: fix setup of nand gpmi clock

colibri-imx6ull: fix setup of nand gpmi clock

NXP used to setup the gpmi clock root from gpmi_clk in early versions
in their downstream BSP. [1]
However on mainline the gpmi clock root was always setup from enfc
since the beginning of the i.MX 6 series SoCs, which is still the same
today. [2]

NXP followed the mainline approach at some point and changed
setup_gpmi_io_clk to setup gpmi clock root from enfc which left faulty
code behind in our board file. [3]

This commit follows the change of NXP as it improves the performance of
the NAND from ~1.2 MiB/s to ~12 MiB/s. [3]

This change was verified to work in recovery-mode and u-boot loaded
from NAND on all four Colibri iMX6ULL SKUs from Toradex.

The frequency used to read the NAND, measured on RE# (Read Enable):
before this patch: 1.4 MHz
after this patch:   22 MHz
in Linux Kernel:    50 MHz

[1] https://source.codeaurora.org/external/imx/uboot-imx/tree/arch/arm/cpu/armv7/mx6/clock.c?h=nxp/imx_v2016.03_4.1.15_2.0.0_ga#n62
[2] commit 23608e23fd65 ("i.mx: add the initial support for freescale i.MX6Q processor")
[3] https://source.codeaurora.org/external/imx/uboot-imx/commit/?id=7a82a19ceabfb04bbc1591a67c99751748781c7d

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>