target/nrf51: use PAN #16 workaround in reset-init only

After 'reset run' or 'reset halt' the loaded application
is expected to manipulate RAMON register to workaround
the known silicon errata.

Moreover, writing to RAMON register from 'reset-end' event
after 'reset run' may collide with application intentions.

Use the workaround in 'reset-init' event only to ensure
correct function of target algorithms.

Change-Id: I7d2d92e6805a05a83676edb46b3163ef39b9a7e4
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/8104
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/target/nrf51.cfg

@@ -58,4 +58,4 @@ proc enable_all_ram {} {
 	# resetting we enable all banks via the RAMON register
 	mww 0x40000524 0xF
 }
-$_TARGETNAME configure -event reset-end {  enable_all_ram }
+$_TARGETNAME configure -event reset-init {  enable_all_ram }