lib: sbi: Synchronize PMP settings with virtual memory system

As per section 3.7.2 of RISC-V Privileged Specification,
PMP settings must be synchronized with the virtual memory
system after PMP settings have been written.

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>