lib: sbi: Add functions to map/unmap shared memory

When Smepmp is enabled, M-mode will need to map/unmap the
shared memory before it can read/write to it. This patch
adds functions to create dynamic short-lived mappings.

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