ofwfb: fix vga/hdmi console with ASMEDIA during boot on powerpc64(le)

On recent OpenBMC firmware, the onboard ASMEDIA video card framebuffer
address was removed from device tree for security purposes (value is set
to zero to avoid leaking the address).

This patch works around the problem by taking framebuffer base address
from the "ranges" property of a parent node.

Reviewed by: luporl, jhibbits (on IRC)
MFC after: 2 weeks
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D30626