staging: mt7621-pinctrl: use pinconf-generic for 'dt_node_to_map' and 'dt_free_map'

staging: mt7621-pinctrl: use pinconf-generic for 'dt_node_to_map' and 'dt_free_map'

Instead of reimplement afunction to do 'dt_node_to_map' task like
'rt2880_pinctrl_dt_node_to_map' make use of 'pinconf_generic_dt_node_to_map_all'
generic function for this task. Also use its equivalent function for free which
is 'pinconf_generic_dt_free_map'. Remove 'rt2880_pinctrl_dt_node_to_map' function
which is not needed anymore. This decrease a bit driver LOC and make it more
generic. To properly compile this changes 'GENERIC_PINCONF' option is selected
with the driver in its Kconfig file.

This also fix a problem with function 'rt2880_pinctrl_dt_node_to_map' which was
calling internally 'pinctrl_utils_reserve_map' which expects 'num_maps' to be initialized.
It does a memory allocation based on the value, and triggers the following
warning if is not properly set:

if (unlikely(order >= MAX_ORDER)) {
        WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
        return NULL;
}

Generic function 'pinconf_generic_dt_node_to_map_all' initializes properly
'num_maps' to zero fixing the problem.

Fixes: e12a1a6e087b ("staging: mt7621-pinctrl: refactor rt2880_pinctrl_dt_node_to_map function")
Reported-by: NeilBrown <neil@brown.name>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>