backports: add of_find_property_value_of_size()

Upstream commit: b3cb62500ef43511480f416f957cb6d60c24984b

commit 2e98a32a274274fca0e6e ("backport: add
of_property_read_u64_array()") added a call
to of_find_property_value_of_size(), which is a
static function, causing compilation error.

Implement it locally as well.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
[remove of_find_property_value_of_size() from backport-3.10.c]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>