target/arm: Make FPSCR.LTPSIZE writable for MVE

target/arm: Make FPSCR.LTPSIZE writable for MVE

The M-profile FPSCR has an LTPSIZE field, but if MVE is not
implemented it is read-only and always reads as 4; this is how QEMU
currently handles it.

Make the field writable when MVE is implemented.

We can safely add the field to the MVE migration struct because
currently no CPUs enable MVE and so the migration struct is never
used.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210520152840.24453-8-peter.maydell@linaro.org