riscv: add support to check machine length at runtime

Highest two bits of misa can be used to check machine length. Add code
to support this.

Change-Id: I3bab301d38ea8aabf2c70437e179287814298b25
Signed-off-by: Xiang Wang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/27770
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/arch/riscv/include/arch/cpu.h

@@ -51,5 +51,11 @@ static inline int supports_extension(char ext)
 	return read_csr(misa) & (1 << (ext - 'A'));
 }
 
+static inline int machine_xlen(void)
+{
+	int mxl = (read_csr(misa) >> (__riscv_xlen - 2)) & 3;
+	return (1 << mxl) * 16;
+}
+
 struct cpu_info *cpu_info(void);
 #endif /* __ARCH_CPU_H__ */