stmmac: fix missing MODULE_LICENSE in stmmac_platform

Commit 50649ab1 ("stmmac: drop driver from stmmac platform code")
was a bit overzealous in removing code and dropped the MODULE_*
macro's that are still needed since stmmac_platform can be a module.
Fix this by putting the macro's remvoed in 50649ab1 back.

This fixes the following errors when used as a module:
  stmmac_platform: module license 'unspecified' taints kernel.
  Disabling lock debugging due to kernel taint
  stmmac_platform: Unknown symbol devm_kmalloc (err 0)
  stmmac_platform: Unknown symbol stmmac_suspend (err 0)
  stmmac_platform: Unknown symbol platform_get_irq_byname (err 0)
  stmmac_platform: Unknown symbol stmmac_dvr_remove (err 0)
  stmmac_platform: Unknown symbol platform_get_resource (err 0)
  stmmac_platform: Unknown symbol of_get_phy_mode (err 0)
  stmmac_platform: Unknown symbol of_property_read_u32_array (err 0)
  stmmac_platform: Unknown symbol of_alias_get_id (err 0)
  stmmac_platform: Unknown symbol stmmac_resume (err 0)
  stmmac_platform: Unknown symbol stmmac_dvr_probe (err 0)

Fixes: 50649ab1

 ("stmmac: drop driver from stmmac platform code")
Reported-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>