ARM: integrator: ensure ap_syscon_base is initialised when !CONFIG_MMU

When running on Integrator/AP using atags, ap_syscon_base is initialised
in ->map_io, which isn't called for !MMU platforms.

Instead, initialise the pointer in ->machine_init, as we do when booting
with device-tree.

Cc: stable@vger.kernel.org
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>