gpio: stmfx: add function stmfx_read_reg and stmfx_write_reg

Add the helper functions stmfx_read_reg() and stmfx_write_reg() to avoid
duplicated code for access to stmfx's register with mask.

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