Currently, the Renesas pin control driver supports pins that can switch
their I/O voltage levels between either 1.8V and 3.3V, or between 2.5V
and 3.3V.  However, some SoCs have pins that can switch between 1.8V and
2.5V.

Add support for this by replacing the separate SH_PFC_PIN_CFG_IO_VOLTAGE
capability and voltage level flags by a 2-bit field, to cover three
possible I/O voltage switching options.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/0c04925457bf3f7e78e7e3851528d9a4c29246da.1678271030.git.geert+renesas@glider.be