lib: sbi: Update pmp_get() to return decoded size directly

Currently pmp_get() returns the log2 length of the PMP memory
region size. The caller has to calculate the size based on that
and the same codes are duplicated.

Update this function to return decoded size directly.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>