The I2C pin functions in R_PIO used to be named "s_twi".

As we usually use the name "i2c" instead of "twi" in the mainline
kernel, change these names to "s_i2c" for consistency.

The "s_twi" functions are not yet referenced by any device trees in
mainline kernel so I think it's safe to change the name.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>