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The ISI is only present on the at91sam9g25, move the definition to the
at91sam9g25ek board dts to avoid warnings.

Solves the following warning for other 9x5ek boards:
Warning (unit_address_vs_reg): Node /ahb/apb/isi@f8048000 has a unit name, but no reg property
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>