board: v2m_beetle: drop CONFIG_PINMUX=y

There's no actual pinmux subsystem driver for v2m_beetle, and
CONFIG_PINMUX is only used as a proxy to build pinmux.c.

Drop the config and change the file to be built unconditionally (same as
mps2_an521 and mps2_an385.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>