MIPS: TXx9: remove useless RTC definitions

MIPS: TXx9: remove useless RTC definitions

The RTC definitions were moved to the driver, remove them from the platform
header.

[paul.burton@mips.com:
  - Also remove the unused tx4939_rtcptr which would use struct
    tx4939_rtc_reg if it were ever expanded.]

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20024/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org