[ARM] remove memzero()

As suggested by Andrew Morton, remove memzero() - it's not supported
on other architectures so use of it is a potential build breaking bug.
Since the compiler optimizes memset(x,0,n) to __memzero() perfectly
well, we don't miss out on the underlying benefits of memzero().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>