Documentation: binding: Modify the exynos5440 PCIe binding

According to using PHY framework, updates the exynos5440-pcie binding.  For
maintaining backward compatibility, leaves the current dt-binding.  (It
should be deprecated.)

Recommends to use the PHY Framework and "config" property to follow the
designware-pcie binding.  If you use the old way, can see "missing *config*
reg space" message.  Because the getting configuration space address from
range is old way.

NOTE: When use the "config" property, first name of 'reg-names' must be set
to "elbi".  Otherwise driver can't maintain the backward capability.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>