PCI: pci-bridge-emul: Add support for PCIe extended capabilities

Add support for PCIe extended capabilities, which we just redirect to the
emulating driver.

[pali: Fix writing new value with W1C bits]
Link: https://lore.kernel.org/r/20220222155030.988-3-pali@kernel.org


Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>