ARM: mxs: don't select HAVE_PWM

The HAVE_PWM symbol is only for legacy platforms that provide
the PWM API without using the generic framework. MXS actually
uses that framework, and selecting the symbol anyway might
cause build errors like

drivers/built-in.o: In function `pwm_beeper_resume':
:(.text+0x1f4fc0): undefined reference to `pwm_config'
:(.text+0x1f4fc8): undefined reference to `pwm_enable'
drivers/built-in.o: In function `pwm_beeper_suspend':
:(.text+0x1f4ffc): undefined reference to `pwm_disable'

when CONFIG_PWM is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Shawn Guo <shawn.guo@linaro.org>