riscv: Add kconfig option to run U-Boot in S-mode

This patch adds kconfig option RISCV_SMODE to run U-Boot in
S-mode. When this opition is enabled we use s<xyz> CSRs instead
of m<xyz> CSRs.

It is important to note that there is no equivalent S-mode CSR
for misa and mhartid CSRs so we expect M-mode runtime firmware
(BBL or equivalent) to emulate misa and mhartid CSR read.

In-future, we will have more patches to avoid accessing misa and
mhartid CSRs from S-mode.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>

arch/riscv/include/asm/encoding.h

@@ -7,6 +7,12 @@
 #ifndef RISCV_CSR_ENCODING_H
 #define RISCV_CSR_ENCODING_H
 
+#ifdef CONFIG_RISCV_SMODE
+#define MODE_PREFIX(__suffix)	s##__suffix
+#else
+#define MODE_PREFIX(__suffix)	m##__suffix
+#endif
+
 #define MSTATUS_UIE	0x00000001
 #define MSTATUS_SIE	0x00000002
 #define MSTATUS_HIE	0x00000004