ARM: dts: exynos: Fix invalid node referenced by i2c20 alias in Peach Pit and Pi

After moving all nodes under "soc" node in commit 5d99cc59 ("ARM:
dts: exynos: Move Exynos5250 and Exynos5420 nodes under soc"), the i2c20
alias in Peach Pit and Peach Pi stopped pointing to proper node:

    arch/arm/boot/dts/exynos5420-peach-pit.dtb: Warning (alias_paths):
        /aliases:i2c20: aliases property is not a valid node (/spi@12d40000/cros-ec@0/i2c-tunnel)
    arch/arm/boot/dts/exynos5800-peach-pi.dtb: Warning (alias_paths):
        /aliases:i2c20: aliases property is not a valid node (/spi@12d40000/cros-ec@0/i2c-tunnel)

Fixes: 5d99cc59 ("ARM: dts: exynos: Move Exynos5250 and Exynos5420 nodes under soc")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>