net: dsa: bcm_sf2: Fix module autoload for OF registration

If the driver is built as a module, autoload won't work because the module
alias information is not filled. So user-space can't match the registered
device with the corresponding module.

Export the module alias information using the MODULE_DEVICE_TABLE() macro.

Before this patch:

$ modinfo drivers/net/dsa/bcm_sf2.ko | grep alias
alias:          platform:brcm-sf2

After this patch:

$ modinfo drivers/net/dsa/bcm_sf2.ko | grep alias
alias:          platform:brcm-sf2
alias:          of:N*T*Cbrcm,bcm7445-switch-v4.0C*
alias:          of:N*T*Cbrcm,bcm7445-switch-v4.0

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>