greybus: build: add pwm to config options to enable

Add PWM to the kernel config options that shall be enabled.

When PWM is not enabled connection init will fail with

	greybus endo0:3:4:9:9: failed to register PWM: -22

when using the default manifest.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>