include: sbi: Remove redundant includes from sbi_platform.h

The sbi_ecall.h and sbi_trap.h are not required to be included in
sbi_platform.h hence we remove it. This way sources requiring
these headers will have to explicitly include it thereby reducing
implicit includes.

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