target-arm: A64: MRS/MSR for FPCR and FPSR access

Add FPCR and FPSR to the set of registers handled by MRS/MSR;
implemented via calls to helper functions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>