include: dt-bindings: gecko: Correct include-guard

Both `include/zephyr/dt-bindings/pinctrl/gecko-pinctrl.h` and
`include/zephyr/dt-bindings/pinctrl/gecko-pinctrl-s1.h` define
`ZEPHYR_INCLUDE_DT_BINDINGS_PINCTRL_GECKO_PINCTRL_H_`
to prevent duplicate inclusion.

Changed it to `ZEPHYR_INCLUDE_DT_BINDINGS_PINCTRL_GECKO_PINCTRL_S1_H_`
in `include/zephyr/dt-bindings/pinctrl/gecko-pinctrl-s1.h` side.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>