dts: rename 'nios2,i2c' compatible to 'altr,nios2-i2c'

The original NIOS-II developer and former vendor is Altera, which is
now part of Intel. Let's not add a new vendor prefix for something
that already exists and has been acquired; move it to use the existing
'altr,' prefix instead.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>