boards: Move default pinmux enable to <board>_defconfig

Most boards enable the pinmux by default by setting CONFIG_PINMUX=y in
<board>_defconfig, but several boards did it in Kconfig.defconfig
instead. Several code reviews, such as #5043, have suggested using a
select in Kconfig.board to enable the pinmux, however this caused
warnings:

warning: (BOARD_LPCXPRESSO54114) selects PINMUX which has unmet direct
dependencies (BOARD_FRDM_K64F || BOARD_FRDM_KL25Z || BOARD_FRDM_KW41Z ||
BOARD_HEXIWEAR_K64 || BOARD_USB_KW24D512 || BOARD_GALILEO)

Fix how these boards enable the pinmux so they are more consistent with
other boards.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>