Add an API to get the next unreserved BAR starting from a given BAR number
that can be used by the endpoint function.

Link: https://lore.kernel.org/r/20210201195809.7342-4-kishon@ti.comSigned-off-by: NKishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>