drivers: wdt_gecko: wdt opt pause in sleep also in EM1 mode

default watchdog initialization disable counting in EM1, EM2 and EM3
modes. If user use the WDT_OPT_PAUSE_IN_SLEEP flag via the watchdog api,
all 3 EM modes must take this flag into account to avoid wdt count
being frozen if we don't want to.

Signed-off-by: Adrien MARTIN <adrienmar@kickmaker.net>