hw/arm_boot.c: Make SMP boards specify address to poll in bootup loop

The secondary CPU bootloader in arm_boot.c holds secondary CPUs in a
pen until the primary CPU releases them. Make boards specify the
address to be polled to determine whether to leave the pen (it was
previously hardcoded to 0x10000030, which is a Versatile Express/
Realview specific system register address).

Signed-off-by: Evgeny Voevodin <e.voevodin@samsung.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
