libc: memset: Fix MISRA issues
libc: memset: Fix MISRA issues

MISRA complained about "0"s not being followed by an "U" (please note
my protest about this!) and about values not being explicitly compared
to 0 (fair enough).
Also use explicit pointer types.

Fix those issues to make the CI happy.

Change-Id: I4d11e49c14f16223a71c78b0fc3e68ba9a1382d3
Signed-off-by: Andre Przywara <94eb15b8f54b8e37c3f8d338296201be47dc5c3e@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware