Correct CPACR reset value for v7 cores

Correct CPACR reset value for v7 cores

In commit f0aff255700 we made cpacr_write() enforce that some CPACR
bits are RAZ/WI and some are RAO/WI for ARMv7 cores. Unfortunately
we forgot to also update the register's reset value. The effect
was that (a) a guest that read CPACR on reset would not see ones in
the RAO bits, and (b) if you did a migration before the guest did
a write to the CPACR then the migration would fail because the
destination would enforce the RAO bits and then complain that they
didn't match the zero value from the source.

Implement reset for the CPACR using a custom reset function
that just calls cpacr_write(), to avoid having to duplicate
the logic for which bits are RAO.

This bug would affect migration for TCG CPUs which are ARMv7
with VFP but without one of Neon or VFPv3.

Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20180522173713.26282-1-peter.maydell@linaro.org