pinconf-generic: clarify pull up and pull down config values

PIN_CONFIG_BIAS_PULL_DOWN and PIN_CONFIG_BIAS_PULL_UP values can
be custom or an SI unit such as ohms

Signed-off-by: Niyas Sait <niyas.sait@linaro.org>
Link: https://lore.kernel.org/r/20221115175415.650690-3-niyas.sait@linaro.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>