MIPS: TXx9: Update rbtx49xx_defconfig

Enable following features:

  * MTD (RBTX4939, NAND_TXX9NDFMC)
  * HW_RANDOM (HW_RANDOM_TX4939)
  * SOUND (SND_SOC_TXX9ACLC)
  * DMADEVICE (TXX9_DMAC)

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