soc: samd51: Fix xosc32 initialization

At osc32k_init in the soc_samd5x.c file the start-up value of 7 which
is reserved. This fixes the startup timeout and control gain with the
correct values.

Signed-off-by: Marcel Birthelmer <marcel@carrietech.com>
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>