of: fix sparse warnings in of_find_next_cache_node

sparse gives a warning that 'handle' is not a __be32:

../drivers/of/base.c:2261:61: warning: incorrect type in argument 1 (different base types)
../drivers/of/base.c:2261:61:    expected restricted __be32 const [usertype] *p
../drivers/of/base.c:2261:61:    got unsigned int const [usertype] *[assigned] handle

We could just change the type, but the code can be improved by using
of_parse_phandle instead of open coding it with of_get_property and
of_find_node_by_phandle.

Signed-off-by: Rob Herring <robh@kernel.org>