mod/power_distributor: Calculate power attributes for distribution

This patch calculates two power attributess, i.e. limits and demands
for power distribution. Traversing the breadth first tree bottom up,
this calculates the attribute of each domain based on
following calculation,

domain limit = min(children limit, own limit)
domain demand = max(children demand, own demand)

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: I4699891fa84438610323eb43c33b7fcd0683301a