pwm: lpc18xx-sct: Reduce number of devm memory allocations

Each devm allocations has an overhead of 24 bytes to store the related
struct devres_node additionally to the fragmentation of the allocator.
So allocating 16 struct lpc18xx_pwm_data (which only hold a single int)
adds quite some overhead. Instead put the per-channel data into the
driver data struct and allocate it in one go.

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