lib: open-amp: Use struct fw_resource_table type instead of void

The type of the resource table is known, casting to and from void* only
hides this type which can prevent the compiler from giving helpful
warnings. One warning would have been the accidental use of
"st_resource_table" in a cast, a struct which does not exist.

Use the fw_resource_table type when dealing with resource tables.

Signed-off-by: Andrew Davis <afd@ti.com>