[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 3957a30)

Author: Damian-Nordic

boot/zephyr/prj.conf

@@ -36,3 +36,4 @@ CONFIG_CBPRINTF_NANO=y
 CONFIG_PICOLIBC=y
 ### Disable malloc arena because we don't need it
 CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0
+CONFIG_NRF_RTC_TIMER_USER_CHAN_COUNT=0