lib: Optimize inline assembly for unprivilege access functions

Currently, unprivileged access functions uses few additional instructions
which are redundant. It also uses specific registers which are not necessary.

Remove those additional instructions and let the compiler choose the
registers.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>