hw/arm/virt: Make PL061 GPIO lines pulled low, not high

hw/arm/virt: Make PL061 GPIO lines pulled low, not high

For the virt board we have two PL061 devices -- one for NonSecure which
is inputs only, and one for Secure which is outputs only. For the former,
we don't care whether its outputs are pulled low or high when the line is
configured as an input, because we don't connect them. For the latter,
we do care, because we wire the lines up to the gpio-pwr device, which
assumes that level 1 means "do the action" and 1 means "do nothing".
For consistency in case we add more outputs in future, configure both
PL061s to pull GPIO lines down to 0.

Reported-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>