hw/misc/allwinner-dramc: Move sysbus_mmio_map call from init -> realize

In order to make the next commit trivial, move the sysbus_init_mmio()
call in allwinner_r40_dramc_init() just before the corresponding
sysbus_mmio_map_overlap() call in allwinner_r40_dramc_realize().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20231019071611.98885-4-philmd@linaro.org>