pinconf-generic: add BIAS_BUS_HOLD pinconf

pinconf-generic: add BIAS_BUS_HOLD pinconf

Add a new PIN_CONFIG_BIAS_BUS_HOLD pin configuration for a bus holder
pin mode (also known as bus keeper, or repeater). This is a weak latch
which drives the last value on a tristate bus. Another device on the bus
can drive the bus high or low before going tristate to change the value
driven by the pin.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
(cherry picked from commit a2df4269cad79635201587c5c5404f0b1cb0b05c)
Signed-off-by: Mark Brown <broonie@linaro.org>