stlink: fix handling of DPv1 and DPv2 banked registers

stlink: fix handling of DPv1 and DPv2 banked registers

Arm DPv1 and DPv2 support banked registers by setting the bank in
field DPBANKSEL of register DP_SELECT.
Old ST-Link firmware don't support banked registers and setting a
bank other than bank zero on DPv1 or DPv2 cause issues in the
firmware because it cannot set back bank zero to read CTRL/STAT.
New ST-Link firmware mask away DPBANKSEL bits while writing in
DP_SELECT but support banked register using the same packed method
used by OpenOCD:
	#define BANK_REG(bank, reg) (((bank) << 4) | (reg))

Add a new macro STLINK_F_HAS_DPBANKSEL for firmware that support
arm DPv1 and DPv2, plus trigger an error if banked registers are
requested on old firmware.
Prevent changing DPBANKSEL on old firmware.
Log a debug message when changing DPBANKSEL will be ignored.

Change-Id: Iaa592517831d63f8da2290db54f6b32504e3081b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4978
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>