rtc: ds1337: drop "SYS" from config variables

There is some inconsistency between uses of CONFIG_RTC_DS13xx and
CONFIG_SYS_RTC_DS13xx. Address this by dropping the "SYS" from
these variables.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>