This helper macro is for declaring single bit (SB) mask pinconf,
and is used to prepare for modifying aspeed_pin_config
structure, the aspeed_pin_config structure @bit variable will be
modified to @mask.

This case is common in the AST2400/AST2500 which the mask is a single bit.
Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-5-andrew@aj.id.auSigned-off-by: Linus Walleij <linus.walleij@linaro.org>