lib: Factor-out SBI legacy extension

This patch factor-out SBI legacy extension into its own source
for better modularity of SBI implementation.

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

lib/sbi/objects.mk

@@ -14,6 +14,7 @@ libsbi-objs-y += riscv_locks.o
 
 libsbi-objs-y += sbi_console.o
 libsbi-objs-y += sbi_ecall.o
+libsbi-objs-y += sbi_ecall_legacy.o
 libsbi-objs-y += sbi_emulate_csr.o
 libsbi-objs-y += sbi_fifo.o
 libsbi-objs-y += sbi_hfence.o