PCI: dwc: artpec6: Split artpec6_pcie_establish_link() into smaller functions

Split artpec6_pcie_establish_link() into smaller functions
to better match other drivers such as dra7xx and imx6.
This is also done to prepare for endpoint mode support.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>