mfd: da9063: Register RTC only on DA9063L

The DA9063L does not contain RTC block, unlike the full DA9063.
Split the RTC block into separate mfd cell and register it only
on DA9063.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>