phy: rockchip: snps-pcie3: Fix bifurcation for RK3588

Misconfigured `PHP_GRF_PCIESEL` values are causing bifurcation issues,
for example on the FriendlyElec CM3588 NAS board which uses bifurcation
on both PCIe PCIe ports (all four lanes) to enable four M.2 NVMe
sockets. Without this fix, NVMe devices do not get recognized.

Correct the `PHP_GRF_PCIESEL` register configuration and simplify the
bifurcation logic, enabling proper PCIe bifurcation based on the
data-lanes property.

This fix is adapted from the upstream Linux commit by Michal Tomek:
f8020dfb311d ("phy: rockchip-snps-pcie3: fix bifurcation on rk3588")

Fixes: 50e54e80 ("phy: rockchip: snps-pcie3: Add support for RK3588")
Signed-off-by: Sebastian Kropatsch <seb-dev@mail.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>