ARM: dts: stm32: Remove useless "st, stm32-gpio" compatible string

Since pinctrl_stm32 driver update, each gpio bank is now binded
by pinctrl driver. The compatible string "st,stm32-gpio" becomes
useless, remove it.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>