leds: lp55xx: use common clock framework when external clock is used

Program execution is timed with 32768Hz clock in the LP55xx family devices.
To run LED functionalities, LP55xx devices provide two options.
One is using internal clock. The other is using external clock.
This patch enables external clock detection automatically.
If external clock is not detected, then the internal clock will be used in the
LP55xx driver.

Valid clock rate is 32768Hz in LP55xx devices.

This new API is used in each LP55xx driver like LP5521 and LP5562.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>