mailmap: Update email for Punit Agrawal

As I'll no longer be working with Arm, add a mailmap entry so any mail
directed towards me reaches the appropriate mailbox.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>