libfdt: Make fdt_get_max_phandle() an inline

It's now a trivial wrapper around fdt_find_max_phandle() so we might as
well inline it.  We also remove it from the versioning linker script.
Theoretically, that's a breaking ABI change except that we haven't yet
released a version with it exposed in the shared object, so we can get
away with it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>