libfdt: return correct value if #size-cells property is not present

According to the device tree specification, the default value for
was not present.

This patch also makes fdt_address_cells() and fdt_size_cells() conform
to the behaviour documented in libfdt.h. The defaults are only returned
if fdt_getprop() returns -FDT_ERR_NOTFOUND, otherwise the actual error
is returned.

This is based on upstream commit:
aa7254d ("libfdt: return correct value if #size-cells property is not present")
but misses the test case part, as we don't implement them in U-Boot.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>