platform: Remove dummy system reset functions

Few platforms have dummy system reset functions so let's remove
these dummy system reset functions to allow generic code deal
with it in the right way.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>