FDT: introduce global phandle allocation

Allocating an FDT phandle (a unique identifier) using a static
variable in a static inline function defined in a header file works
only if all users are in the same source file. So trying to allocate
a handle from two different compilation units fails.
Introduce global phandle allocation and reference code to properly
allocate unique phandles.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>