lib: sbi_hsm: Rename 'priv' argument to 'arg1'

'priv' argument of sbi_hsm_hart_start() and sbi_hsm_hart_suspend()
may mislead people to think it stands for 'privilege mode', but it
is not. Change it to 'arg1' to clearly indicate the a1 register.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>