[nrf noup] zephyr: 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 <[email protected]>
Signed-off-by: Torsten Rasmussen <[email protected]>
Signed-off-by: Jamie McCrae <[email protected]>
Signed-off-by: Dominik Ermel <[email protected]>
(cherry picked from commit 9fa2c07)

Author: Damian-Nordic

boot/zephyr/prj.conf

@@ -35,3 +35,4 @@ CONFIG_MCUBOOT_LOG_LEVEL_INF=y
 CONFIG_CBPRINTF_NANO=y
 ### Use the minimal C library to reduce flash usage
 CONFIG_MINIMAL_LIBC=y
+CONFIG_NRF_RTC_TIMER_USER_CHAN_COUNT=0