riscv: save hart ID and device tree passed by prior boot stage

Store the hart ID and device tree passed by the prior boot stage (in a0
and a1) in registers s0 and s1. Replace one use of s1 in start.S to
avoid overwriting it.

The device tree is also stored in memory to make it available to U-Boot
with the configuration CONFIG_OF_PRIOR_STAGE.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
index ae57fb8..d9f820c 100644
--- a/arch/riscv/cpu/cpu.c
+++ b/arch/riscv/cpu/cpu.c
@@ -6,6 +6,12 @@
 #include <common.h>
 #include <asm/csr.h>
 
+/*
+ * prior_stage_fdt_address must be stored in the data section since it is used
+ * before the bss section is available.
+ */
+phys_addr_t prior_stage_fdt_address __attribute__((section(".data")));
+
 enum {
 	ISA_INVALID = 0,
 	ISA_32BIT,