ARM: 8223/1: Add unwinding support for __memzero function

The __memzero function never had unwinding annotations added.
Currently, when accessing invalid pointer by __memzero occurs the
backtrace shown will stop at __memzero or some completely unrelated
function. Add unwinding annotations in hopes of getting a more
useful backtrace in following cases:
1. die on accessing invalid pointer by __memzero
2. kprobe trapped at any instruction within __memzero
3. interrupted at any instruction within __memzero

Signed-off-by: Lin Yongting <linyongting@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>