arm, at91: add icache support

add at least icache support for at91 based boards.
This speeds up NOR flash access on an at91sam9g15
based board from 15.2 seconds reading 8 MiB from
a SPI NOR flash to 5.7 seconds.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>