clocksource/drivers/bcm2835: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. Also drop the FSF address.

Cc: Simon Arlott <simon@arlott.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>