lib: Create a separate math helper function file

There may be few common mathematics helper functions which can be used
anywhere in OpenSBI project.

Add a separate math helper function file to add these functions.

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

lib/sbi/objects.mk

@@ -24,6 +24,7 @@ libsbi-objs-y += sbi_ecall_vendor.o
 libsbi-objs-y += sbi_emulate_csr.o
 libsbi-objs-y += sbi_fifo.o
 libsbi-objs-y += sbi_hart.o
+libsbi-objs-y += sbi_math.o
 libsbi-objs-y += sbi_hfence.o
 libsbi-objs-y += sbi_hsm.o
 libsbi-objs-y += sbi_illegal_insn.o