rtc: imxdi: Convert to a DT-only driver

i.MX has been converted to a DT-only platform, so make the driver
depend on OF, remove the CONFIG_OF ifdefery and remove of_match_ptr().

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210315172029.173250-1-festevam@gmail.com