exynos: Rework legacy PWM usage

exynos: Rework legacy PWM usage

The way that the timer support is currently done for exynos/nexell
platforms relies on the legacy PWM infrastructure, and that needs to be
updated. However, we really cannot safely undef CONFIG_DM_PWM to build
the timer.c file without warnings. For now, rename the relevant legacy
functions to be prefixed with s5p_ and add prototypes to the arch pwm.h
files.

Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Dzmitry Sankouski <dsankouski@gmail.com>
Cc: Stefan Bosch <stefan_b@posteo.net>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>