pinctrl: sprd: Add pin high impedance mode support

For Spreadtrum pin controller, it will be the high impedance
mode if disable input and output mode for a pin. Thus add
PIN_CONFIG_BIAS_HIGH_IMPEDANCE configuration to support it.

Signed-off-by: Linhua Xu <linhua.xu@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/3bdac4c2673b54c940e511f3fa569ee33b87b8d5.1585124562.git.baolin.wang7@gmail.com


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