lib: sbi: Fix PMP granularity handling in sbi_hart_map_saddr()

The sbi_hart_map_saddr() must create PMP mapping of size greater
than or equal to PMP granularity otherwise PMP mapping does not
work when size parameter less than sbi_hart_pmp_granularity(scratch).

Fixes: 6e44ef68

 ("lib: sbi: Add functions to map/unmap shared memory")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>