ipq806x: fixup nbg6817 internal mmc and switch configuration in DTS

The setting mmc-ddr-1_8v in the platform dts leads to read errors. The
device is unusable and system reboots in a loop. Because NBG6817 is the
only mmc device, I removed it in base dts.

The second change removes settings now present in base dts.

The third change references was a wrong conversion of constants in the switch settings.
Switch now initializes again.

Signed-off-by: André Valentin <avalentin@marcant.net>