hw/vexpress.c: Move secondary CPU boot code to SRAM

On real Versatile Express hardware, the boot ROM puts the secondary
CPU bootcode/holding pen in SRAM. We can therefore rely on Linux not
trashing this memory until secondary CPUs have booted up, and can
put our QEMU-specific pen code in the same place. This allows us to
drop the odd "hack" RAM page we were using before.

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