AT91: atmel_pwm only available for certain AT91 processors

Only three of Atmel's AT91 processors (SAM9263, SAM9RL and CAP9) include a
PWM controller.

It should therefore only be possible to enable the misc/atmel_pwm.c driver
on those processors (and not all AT91 processors).

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>