target/cortex_m: add DSCSR_CDSKEY bit definition

Needed e.g. for flash drivers handling secure mode.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Change-Id: If6cb49609140d06a73bcf2e446b6a634d6326e80
Reviewed-on: https://review.openocd.org/c/openocd/+/8435
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/cortex_m.h

@@ -150,6 +150,7 @@ struct cortex_m_part_info {
 #define VC_CORERESET	BIT(0)
 
 /* DCB_DSCSR bit and field definitions */
+#define DSCSR_CDSKEY	BIT(17)
 #define DSCSR_CDS		BIT(16)
 
 /* NVIC registers */