bus: mvebu-bus: Provide inline stub for mvebu_mbus_get_dram_win_info

In preparation for allowing CONFIG_MVNETA_BM to build with COMPILE_TEST,
provide an inline stub for mvebu_mbus_get_dram_win_info().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>