lmb: riscv: Add arch_lmb_reserve()

Add arch_lmb_reserve() implemented using arch_lmb_reserve_generic().
It is rather likely this architecture also needs to cover U-Boot with
LMB before booting Linux.

Reviewed-by: Rick Chen <rick@andestech.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Atish Patra <atish.patra@wdc.com>
Cc: Leo <ycliang@andestech.com>
Cc: Rick Chen <rick@andestech.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tom Rini <trini@konsulko.com>

arch/riscv/lib/bootm.c

@@ -135,3 +135,16 @@ int do_bootm_vxworks(int flag, int argc, char *const argv[],
 {
 	return do_bootm_linux(flag, argc, argv, images);
 }
+
+static ulong get_sp(void)
+{
+	ulong ret;
+
+	asm("mv %0, sp" : "=r"(ret) : );
+	return ret;
+}
+
+void arch_lmb_reserve(struct lmb *lmb)
+{
+	arch_lmb_reserve_generic(lmb, get_sp(), gd->ram_top, 4096);
+}