refactor(st-gpio): code improvements

No functional, change, but some improvements:
- Declare set_gpio() as static (only called locally)
- Handle the type ('open-drain') property independently from the
  mode one.
- Replace mmio_clrbits_32() +  mmio_setbits_32() with
  mmio_clrsetbits_32().
- Add a missing log
- Add missing U() in macro definitions

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Change-Id: I1a79609609ac8e8001127ebefdb81def573f76fa