ARM: mx25: Remove imxdi_rtc platform code

platform-imxdi_rtc.c is only used by mx25, so it can safely go away now that
mx25 has been converted to dt.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>