pci-bridge-emul driver already allocates buffer for capabilities up to the
PCI_EXP_SLTSTA2 register, but does not define bit access behavior for these
registers. Add these missing definitions.

Link: https://lore.kernel.org/r/20211130172913.9727-3-kabel@kernel.orgSigned-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>