target/arm/cpu: Allow init-svtor property to be set after realize

target/arm/cpu: Allow init-svtor property to be set after realize

Make the M-profile "init-svtor" property be settable after realize.
This matches the hardware, where this is a config signal which
is sampled on CPU reset and can thus be changed between one
reset and another. To do this we have to change the API we
use to add the property.

(We will need this capability for the SSE-200.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190219125808.25174-4-peter.maydell@linaro.org