pinmux: stm32l4x rework header definition

For a better clarity and minimize the possibility
of false definitions and duplicates, gather defines
by port and store the by alphabetical order then in
alternate function order.

Change-Id: Ib9febc9e6c5037a774190007120b87bf100c3fca
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>