fdt: Tidy up a few fdtdec problems

This fixes five trivial issues in fdtdec.c:
1. fdtdec_get_is_enabled() doesn't really need a default value
2. The fdt must be word-aligned, since otherwise it will fail on ARM
3. The compat_names[] array is missing its first element. This is needed
only because the first fdt_compat_id is defined to be invalid.
4. Added a header prototype for fdtdec_next_compatible()
5. Change fdtdec_next_alias() to only increment its 'upto' parameter
on success, to make the display error messages in the caller easier.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>