xen/arm: WORKAROUND Support kick cpus and switch to hypervisor for the exynos5

xen/arm: WORKAROUND Support kick cpus and switch to hypervisor for the exynos5

Use machine ID to know what is the current board. This value is only given
to the first CPU by the bootloader.

When the exynos 5 starts, there is only one CPU up. Xen needs to start the
secondary cpu. The latter boots in secure mode.

Theses modifications aim to be removed as soon as possible. It should
be moved either in a platform specific boot-wrapper (which will be start
before Xen), or in the bootloader (assuming U-Boot/Grub will support SMP).

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>