soc: atmel: sam4l: Enable RC32K osc

Enable sam4l internal factory calibrated RC32K clock source.  The RC32K
was used as source for Generic Clock 5 using 32 as divider.  The output
is a 1024 Hz clock that can be used by GLOC and TC0 peripherals.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>