[ARM] S3C24XX: Fix use of CONFIG_S3C24XX_PWM

CONFIG_S3C24XX_PWM was defined in arch/arm/plat-s3c24xx/Kconfig but
not used anywhere else as the corresponding makefile used
CONFIG_HAVE_PWM (selected by CONFIG_S3C24XX_PWM) to compile the PWM
driver.

Change the makefile to use CONFIG_S3C24XX_PWM to compile this driver
to ensure it is only build when needed.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>