MIPS: TXx9: Fix Kconfig warnings

If TTY and SND is not n, we got this warnings:

WARNING: unmet direct dependencies detected for HAS_TXX9_SERIAL
  Depends on [n]: TTY [=n] && HAS_IOMEM [=y]
  Selected by [y]:
  - SOC_TX3927 [=y]

WARNING: unmet direct dependencies detected for HAS_TXX9_SERIAL
  Depends on [n]: TTY [=n] && HAS_IOMEM [=y]
  Selected by [y]:
  - SOC_TX4938 [=y]

Only dependencies is enabled, they can be enabled, so
use 'imply' instead of 'select' to fix this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>