lib: sbi: Fixed memory permission check in sbi_dbtr_setup_shmem

The previous code detected shmem_phys_hi and shmem_phys_lo as two
addresses. fix this bug

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>