ipq806x: reorganize nand_pins mux definition

ipq806x: reorganize nand_pins mux definition

It's wrong set the mux to bias-disable. The best way to
do this is by creating a separate group and disable the
specific pins.

By documentation, any subgroup with no bias definition
is ignored so the mux definition is useless.
Rework the definition by sremoving the mux subgroup and
set the remaining subgroup with the mux function and
drive-strength

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>