ARM: dts: exynos: Define fixed regulators in root node for consistency in SMDK5420

Remove the regulators node and define fixed regulators directly under
the root node.  This makes SMDK5420 board consistent with other Exynos
boards.

Name the fixed regulator nodes consistently.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>