ARM: dts: STiH41x-b2020: Update gpio specifier

- Remove useless gpio-cells
- Update second parameter by using GPIO_ACTIVE_HIGH/LOW
  instead of hardcoded value

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>