dt-bindings: watchdog: samsung-wdt: Split if:then: and constrain more

Binding defined two if:then: blocks covering different conditions but
not fully constraining the properties per each variant:
1. "if:" to require samsung,syscon-phandle,
2. "if:" with "else:" to narrow number of clocks and require or disallow
samsung,cluster-index.

This still did not cover following cases:
1. Disallow samsung,syscon-phandle when not applicable,
2. Narrow samsung,cluster-index to [0, 1], for SoCs with only two
clusters.

Solving this in current format would lead to spaghetti code, so re-write
entire "if:then:" approach into mutually exclusive cases so each SoC
appears only in one "if:" block. This allows to forbid
samsung,syscon-phandle for S3C6410, and narrow samsung,cluster-index
to [0, 1].

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>