Revert "of: reserved-memory: Fix using wrong number of cells to get property 'alignment'"

This reverts commit 267b21d0.

Turns out some DTs do depend on this behavior. Specifically, a
downstream Pixel 6 DT. Revert the change at least until we can decide if
the DT spec can be changed instead.

Cc: stable@vger.kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>