rtc: ab8500: Drop AB8540 support

The AB8540 was an evolved version of the AB8500, but it was never
mass produced or put into products, only reference designs exist.
The upstream support was never completed and it is unlikely that
this will happen so drop the support for now to simplify
maintenance of the AB8500.

Cc: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>