ARM: dts: at91: move isi definition to at91sam9g25ek

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>