PCI: mvebu: Drop unused PCI express capability code

Commit dc0352ab ("PCI: mvebu: Add PCI Express root complex
capability block") added support for emulating the PCI Express
capability block. As part of this, the pcie_sltcap, pcie_devctl and
pcie_rtctl fields were added to the mvebu_sw_pci_bridge structure, and
used when reading the corresponding PCI Express capability block
registers.

However, those structure members are never set to any value other than
zero. This makes them unneeded because:

 - pcie_devctl is used to OR *value, so with pcie_devctl always zero,
   it has no effect.

 - for pcie_sltcap and pcie_rtstl, the mvebu_sw_pci_bridge_read()
   function always returns 0 for registers that are not explicitly
   handled.

In preparation for reworking the PCI bridge emulation logic in
pci-mvebu, let's simplify the code by dropping those structure
members.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>