arm64: dts: exynos: Add compatibles to sysreg nodes

System register nodes, implementing syscon binding, should use
appropriate compatible.  This fixes dtbs_check warnings:

  arch/arm64/boot/dts/exynos/exynos5433-tm2.dt.yaml: syscon@13b80000:
    compatible: ['syscon'] is not valid under any of the given schemas

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Link: https://lore.kernel.org/r/20200829142501.31478-8-krzk@kernel.org