[ARM] 5078/1: pxa-pwm: Add missing MODULE_LICENSE to be able to build the driver

 as a module

Without a GPL-compatible license this driver cannot be built as a module,
because the platform_driver_* API is only exported to GPL modules.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>