cortex_m: add detection of MVE feature for Armv8.1-M cores

For Armv8.1-M based cores, detect if the core implements the optional
M-profile vector extension (MVE), using MVFR1 register.

While at there rework armv7m->fp_feature detection based on MVFR0
and MVFR1 registers.

Change-Id: I92d5b1759aea9f7561d285f46acdec51d6efb7b4
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6950
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/target/armv7m.h

@@ -211,6 +211,8 @@ enum {
 	FPV4_SP,
 	FPV5_SP,
 	FPV5_DP,
+	FPV5_MVE_I,
+	FPV5_MVE_F,
 };
 
 #define ARMV7M_NUM_CORE_REGS (ARMV7M_CORE_LAST_REG - ARMV7M_CORE_FIRST_REG + 1)