kbuild: centralize .dts->.dtb rule

All architectures that use cmd_dtc do so in almost the same way. Create
a central build rule to avoid duplication. The one difference is that
most current uses of dtc build $(obj)/%.dtb from $(src)/dts/%.dts rather
than building the .dtb in the same directory as the .dts file. This
difference will be eliminated arch-by-arch in future patches.

MIPS is the exception here; it already uses the exact same rule as the
new common rule, so the duplicate is removed in this patch to avoid any
conflict. arch/mips changes courtesy of Ralf Baechle.

Update Documentation/kbuild to remove the explicit call to cmd_dtc from
the example, now that the rule exists in a centralized location.

Cc: Arnd Bergmann <[email protected]>
Cc: [email protected]
Cc: Olof Johansson <[email protected]>
Cc: Russell King <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Cc: Jonas Bonn <[email protected]>
Cc: [email protected]
Cc: Aurelien Jacquiot <[email protected]>
Cc: [email protected]
Cc: Mark Salter <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: [email protected]
Cc: Chris Zankel <[email protected]>
Cc: [email protected]
Cc: Max Filippov <[email protected]>
Signed-off-by: Ralf Baechle <[email protected]>
Signed-off-by: Stephen Warren <[email protected]>
Signed-off-by: Rob Herring <[email protected]>

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