[ARM] 4809/2: ixp4xx: Merge dsmg600-power.c into dsmg600-setup.c

There is no reason to have power control in a separate file from the
board setup code.  Merge it back into the board setup file and remove
superfluous header includes.

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Signed-off-by: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>