pwm: bcm2835: Allow building for ARCH_BRCMSTB

BCM7211 is supported using ARCH_BRCMSTB and uses this PWM controller
driver, make it possible to build it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>