of: address: Store number of bus flag cells rather than bool

It is more useful to know how many flags cells a bus has rather than
whether a bus has flags or not as ultimately the number of cells is the
information used. Replace 'has_flags' boolean with 'flag_cells' count.

Acked-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20231026135358.3564307-2-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>