hw/vexpress.c: Allow >4GB of RAM for Cortex-A15 daughterboard

hw/vexpress.c: Allow >4GB of RAM for Cortex-A15 daughterboard

Now that we have LPAE support and can handle passing 64 bit
RAM sizes to Linux via the device tree, we can lift the
restriction in the Versatile Express A15 daughterboard model
on not having more than 2GB of RAM. Allow up to 30GB, which
is the maximum that can fit in the address map before running
into the (unmodelled) aliases of the first 2GB.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>