PCI: mobiveil: Add 8-bit and 16-bit CSR register accessors

There are some 8-bit and 16-bit registers in PCIe configuration
space, so add these accessors accordingly.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Reviewed-by: Subrahmanya Lingappa <l.subrahmanya@mobiveil.co.in>
Reviewed-by: Andrew Murray <amurray@thegoodpenguin.co.uk>