pinmux: stm32f4: Compile out unused pinmux

It is useless to include the pinmux for a peripheral if it is not
enabled in the Kconfig. This is unnecessary and it increases the size of
the binary.

Define macros that will default to void if the associated Kconfig is not
enabled.

Change-Id: I0857fcef335c75b8bb6d537fd859f93d5be4a228
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>