lib: sbi: Map/Unmap debug console shared memory buffers

With Smepmp enabled, it is necessary for shared memory from
S/U mode to be mapped/unmapped before and after read/write
of the memory region. This patch maps the debug console
shared memory before accessing it.

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