hw/arm/boot: fix SVE for EL3 direct kernel boot

When booting a CPU with EL3 using the -kernel flag, set up CPTR_EL3 so
that SVE will not trap to EL3.

Signed-off-by: Rémi Denis-Courmont <remi.denis.courmont@huawei.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201030151541.11976-1-remi@remlab.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>