cortex_m: support control.FPCA

Bit 2 of control register is used if the processor includes the FP
extension

Change-Id: Ie21bc9de8cae5bad9d841e1908eff3aa0bb29d4b
Signed-off-by: Sylvain Chouleur <schouleur@graimatterlabs.ai>
Reviewed-on: http://openocd.zylin.com/5853
Reviewed-by: Sylvain Chouleur <sylvain.chouleur@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>