lib: sbi: Introduce simple heap allocator

We provide simple heap allocator to manage the heap space provided
by OpenSBI firmware and platform.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

lib/sbi/objects.mk

@@ -59,6 +59,7 @@ libsbi-objs-y += sbi_domain.o
 libsbi-objs-y += sbi_emulate_csr.o
 libsbi-objs-y += sbi_fifo.o
 libsbi-objs-y += sbi_hart.o
+libsbi-objs-y += sbi_heap.o
 libsbi-objs-y += sbi_math.o
 libsbi-objs-y += sbi_hfence.o
 libsbi-objs-y += sbi_hsm.o