dt-bindings: pinctrl: Update enum for adding SGPM2 and SGPS2

AST2600 has 2 SGPIO master interfaces one with 128 pins and another one
has 80 pins. It also supports 2 SGPIO slave interfaces.
In the current bindings, there are only SGPM1 and SGPS1 defined in enum,
SGPM2 and SGPS2 should also be added in the bindings.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210525055308.31069-2-steven_lee@aspeedtech.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>