target/cortex_m: cumulate DHCSR sticky bits

DCB DHCSR register contains S_RETIRE_ST and S_RESET_ST bits cleared
on a read.

The change introduces a helper function cortex_m_cumulate_dhcsr_sticky().
Call this function each time DHCSR is read to preserve S_RESET_ST state
in the case of a reset event was detected.

Introduce cortex_m_read_dhcsr_atomic_sticky() convenience helper to
read DHCSR, store it to cortex_m->dcb_dhcsr and cumulate sticky bits.

The cumulated state of S_RESET_ST is read and cleared in cortex_m_poll()

Change-Id: Ib679599f850fd219fb9418c6ff32eed7cf5740da
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6180
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>

src/target/cortex_m.h

@@ -213,6 +213,7 @@ struct cortex_m_common {
 
 	/* Context information */
 	uint32_t dcb_dhcsr;
+	uint32_t dcb_dhcsr_cumulated_sticky;
 	uint32_t nvic_dfsr;  /* Debug Fault Status Register - shows reason for debug halt */
 	uint32_t nvic_icsr;  /* Interrupt Control State Register - shows active and pending IRQ */