armv7m: cache: add invalidate_icache_all() stub

This commit:
https://gitlab.denx.de/u-boot/u-boot/commit/d409c962169bd293e39386d0ddfa64d5222a3be4


causes build failure with ICACHE enabled. This is due to missing
invalidate_icache_all() stub. Let's add empty invalidate_icache_all() in
the case where ICACHE is not enabled.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>