clocksource/drivers/samsung_pwm_timer: Fix endian accessors

Fix the Samsung pwm timer access code to deal with kernels built for big
endian operation.

Signed-off-by: Matthew Leach <matthew@mattleach.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>