arm64: dts: hi6220: Add pl031 RTC support

Add pl031 rtc0 and rtc1 support to hi6220 dtsi

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 and tweaked commit description,
 added rtc1 entry as suggested by Guodong]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>