dt-bindings: EXYNOS: Add exynos-srom device tree binding

This patch adds exynos-srom binding information for SROM Controller
driver on Exynos SoCs. Documentation for new subnode properties,
allowing bank configuration are added based on u-boot implementation,
but heavily reworked.

CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Ian Campbell <ijc+devicetree@hellion.org.uk>
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
[p.fedin: Added SROMc configuration description and fixed SROMc mapping]
Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>