[nrf noup] Restore default RTC user channel count

The default value of CONFIG_NRF_RTC_TIMER_USER_CHAN_COUNT
for nRF52 SOCs has been changed from 0 to 3, but it makes
MCUBoot get stuck on erasing flash pages when swapping two
images. Restore the previous value until the RTC issue is
resolved (see NCSDK-14427)

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit dbcdfae)
(cherry picked from commit f5559b7)
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/zephyr/prj.conf

@@ -34,3 +34,4 @@ CONFIG_LOG_MODE_MINIMAL=y # former CONFIG_MODE_MINIMAL
 CONFIG_LOG_DEFAULT_LEVEL=0
 ### Decrease footprint by ~4 KB in comparison to CBPRINTF_COMPLETE=y
 CONFIG_CBPRINTF_NANO=y
+CONFIG_NRF_RTC_TIMER_USER_CHAN_COUNT=0