[MIPS] TXx9: Miscellaneous build fixes

* Fix build if only RBTX4927 or RBTX4938 was selected.
* Move gpio helpers to generic part.
* Select SOC_TX4938 for RBTX4927/37 board.
* Fix parent of rbtx4938_fpga_resource.

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