ARM: dts: stm32: remove "@" and "_" from stm32f7 pinmux groups

Replace all "@" and "_" by "-" in pinmux groups for stm32f7 family.
This avoid errors when using yaml to check the bindings.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>