include: sbi: Make hartmask pointer const in sbi_hartmask_test_hart()

The sbi_hartmask_test_hart() does not modify hartmask so the
hartmask pointer parameter should be declared const.

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>