rtc: compile date.c if DM_RTC

rtc_to_tm() and rtc_mktime() are required for some RTC drivers, at least
PL031. Without this patch, we also need to enable CONFIG_CMD_DATE even if
we don't want or need this command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>