sunxi: build kernel with sun6i-rtc driver

sunxi: build kernel with sun6i-rtc driver

sun6i-rtc cannot be built as a module and the hardware is only
present in some of the sunxi SoCs, see driver source:
        { .compatible = "allwinner,sun6i-a31-rtc" },
        { .compatible = "allwinner,sun8i-a23-rtc" },
        { .compatible = "allwinner,sun8i-h3-rtc" },
        { .compatible = "allwinner,sun8i-r40-rtc" },
        { .compatible = "allwinner,sun8i-v3-rtc" },
        { .compatible = "allwinner,sun50i-h5-rtc" },
        { .compatible = "allwinner,sun50i-h6-rtc" },

Set CONFIG_RTC_DRV_SUN6I=y in kernel config file for cortexa7 and
cortexa53 subtargets which covers all of the above.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>