phy: miphy28lp: Avoid calling of_get_child_count() multiple times

Currently, of_get_child_count() is called in each iteration of the for loop in
miphy28lp_xlate(). This patch stores the return value of of_get_child_count()
in miphy_dev->nphys and call of_get_child_count() once in miphy28lp_probe().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>