MIPS: remove the HT_PCI config option

This option is always selected from LOONGSON_MACH3X.  Switch to just
seleting PCI from that option and definining LOONGSON_PCIIO_BASE based
on CONFIG_LOONGSON_MACH3X.  PCI already selects PCI_DOMAINS.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>