clk: hi6220: Add RTC clock for pl031

Adds clk support for the pl031 RTC on hi6220

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
[jstultz: Forward ported, tweaked commit description]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>