fdt: introduce fdt_verify_alias_address() and fdt_get_base_address()

Introduce two functions, fdt_verify_alias_address() and
fdt_get_base_address(), which can be used to verify the physical address
of a device in a device tree.

fdt_get_base_address() returns the base address of an SOC or PCI node.

fdt_verify_alias_address() prints a message if the address of a node
specified by an alias does not match the given physical address.

Signed-off-by: Timur Tabi <timur@freescale.com>