riscv: update mtime initialization

Add a interface, which is implemented by SoC.

Change-Id: I5524732f6eb3841e43afd176644119b03b5e5e27
Signed-off-by: Xiang Wang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/28372
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/arch/riscv/include/mcall.h

@@ -61,6 +61,9 @@ typedef struct {
 
 void hls_init(uint32_t hart_id); // need to call this before launching linux
 
+/* This function is used to initialize HLS()->time/HLS()->timecmp  */
+void mtime_init(void);
+
 #endif // __ASSEMBLER__
 
 #endif