The SoC1 SCU provides a bias enable bit per pin in the registers at
0x480 (setting the bit disables the bias; the pull direction is fixed
in silicon) and 2-bit drive strength fields at 0x4C0 selecting 4 mA to
16 mA in 4 mA steps. The pin-to-field mapping of the drive strength
registers is sparse and non-linear, so it is kept in a lookup table
mirroring the Linux driver; pins without an entry reject
drive-strength with -ENOTSUPP.
Support the bias-disable, bias-pull-down, bias-pull-up and
drive-strength properties per pin and per group, and select PINCONF so
the generic pinctrl framework parses them.
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>