rtc: gemini: Add optional clock handling

This makes the Gemini optionally take two clock references to
the PCLK and EXTCLK. As we are adding a clock framework to the
Gemini platform we need to make sure that we get the right
references.

Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>