[MIPS] rbtx4938: Fix secondary PCIC and glue internal NICs

* Fix pci ops for secondary PCIC
* Do not reserve 1MB for PCI MEM region (leave PCIBIOS_MIN_MEM zero)
* Use platform_device to provide ethernet addresses for internal NICs.
  (background: TX49XX SoCs include PCI NIC (TC35815 compatible)
  connected via its internal PCI bus, but the NIC's PROM interface is
  not connected to SEEPROM.  So we must provide its ethernet address
  by another way.)
* Check return value of early_read_config_word()

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>