include: Rename sbi_unpriv.h to riscv_unpriv.h

The sbi_unpriv.h has quite a few load_xyz() and store_xyz() helper
routines based on RISC-V inline assembly for unpriviledged accesses
from M-mode. These helper routines are similar to helper routines
present in riscv_locks.h, riscv_io.h, and riscv_atomic.h so let's
rename sbi_unpriv.h to riscv_unpriv.h.

Signed-off-by: Anup Patel <anup.patel@wdc.com>