hw/intc/armv7m_nvic: Update FPDSCR masking for v8.1M

hw/intc/armv7m_nvic: Update FPDSCR masking for v8.1M

The FPDSCR register has a similar layout to the FPSCR.  In v8.1M it
gains new fields FZ16 (if half-precision floating point is supported)
and LTPSIZE (always reads as 4).  Update the reset value and the code
that handles writes to this register accordingly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-16-peter.maydell@linaro.org