lib: sbi: Fix PMP address bits detection

We should ensure that pmpcfg0.pmp0cfg is set to zero before using
pmpaddr0 CSR for detecting implemented PMP address bits.

Fixes: bf216328

 ("lib: sbi: Detect PMP granularity and number
of address bits")
Signed-off-by: Vasan VS <vasan.vs@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>