[ARM] Orion: correctly load mv2120 RTC driver

After Jean Delvare's change "i2c: Convert most new-style drivers
to use module aliasing" (3760f736716f74bdc62a4ba5406934338da93eb2),
loading rtc-xxx from platform code fails.  Update mv2120-setup.c so
that the driver is loaded correctly.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>