Revert "drivers: pinmux: build as static library"

This reverts commit 43309296.

Fixes: #38403

The referred commit introduced `zephyr_library()` for pinmux drivers but
also resulting in #38403 because several boards has `CONFIG_PINMUX=y`
without selecting any pinmux drivers from `drivers/pinmux` thus
generating the following warning:
> No SOURCES given to Zephyr library: drivers__pinmux
>
> Excluding target from build.

This commit reverts the changes so that this warning disappears.
This results in pinmux drivers from `drivers/pinmux` to be located in
libzephyr.a which is messy, but has been so for a long time, even before
Zephyr 1.14 LTS.

The future pinctrl API will be designed in such a way that this problem
will not occur. Thus the old behavior is acceptable until the transition
to pinctrl API has completed.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>