a10_gpio: Add support for more modern pin configuration

a10_gpio previously accepted only {allwinner,}drive and {allwinner,}pull for
drive/bias setting, while newer DTS is using drive-strength and
bias-{disable,pull-up,pull-down} properties. Accept these properties as
well.

Additionally make bias and drive strength optional rather than required; not
setting them should just indicate that we do not need to configure these
properties.

Tested on: BananaPi-M3 (a83t)
Reviewed by: manu
Approved by: emaste (implicit)
Obtained from: NetBSD (partially)
Differential Revision: https://reviews.freebsd.org/D13284