boards: nuvoton: numaker_m2l31ki: reenable userspace

In commit a30c5731 (tests: drivers: can: api:
support numaker_m2l31ki, 2024-05-31), CONFIG_USERSPACE was disabled for
the numaker_m2l31ki. This fix is valid to support running the CAN API
testsuite, but Zephyr's infrastructure does not properly support
disabling userspace at the board level.

As a temporary workaround, reenable userspace to fix failing CI tests
on this board.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>