stmmac: dwmac-rk: create a new probe function

Create a new probe functions that call the necessary setup
functions. This is done in preparation for a code refactor
in this driver and dropping the common probe function in
stmmac_platform.c.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>