include: sbi_scratch: Set per-HART scratch size to 4KB

Currently, the per-HART scratch size is 256 bytes on RV32 and
512 bytes on RV64. This patch set per-HART scratch size to 4KB
(4096 bytes) for both RV32 and RV64 so that we don't run-out
of scratch space anytime soon.

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