armv7m: add FPU registers support

armv7m: add FPU registers support

This patch adds the fpv4-sp-d16 registers to the armv7m register set.

The work is inspired by Mathias K but takes a different approach:
instead of having both double and single presicion registers in the
cache this patch works only with the doubles and counts on GDB to
split the data in halves whenever needed.

Tested with HLA only (on an STM32F334 disco board).

Currently this patch makes all ARMv7-M targets report an FPU-enabled
target description to GDB. It shouldn't harm if the user is not trying
to access non-existing FPU. However, the plan is to make this depend
on actual FPU presence later.

Change-Id: Ifcc72c80ef745230c42e4dc3995f792753fc4e7a
Signed-off-by: Mathias K <kesmtp@freenet.de>
[fercerpav@gmail.com: rework to fit target description framework]
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/514
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>