riscv: implement the invalidate_icache_* functions

Implement the functions invalidate_icache_range() and
invalidate_icache_all().

RISC-V does not have instructions for explicit cache-control. The
functions in this patch are implemented with the memory ordering
instruction for synchronizing the instruction and data streams. This may
be implemented as a cache flush or invalidate on simple processors,
others may only invalidate the relevant cache lines.

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>

arch/riscv/lib/cache.c

@@ -12,6 +12,16 @@ void flush_dcache_range(unsigned long start, unsigned long end)
 
 void invalidate_icache_range(unsigned long start, unsigned long end)
 {
+	/*
+	 * RISC-V does not have an instruction for invalidating parts of the
+	 * instruction cache. Invalidate all of it instead.
+	 */
+	invalidate_icache_all();
+}
+
+void invalidate_icache_all(void)
+{
+	asm volatile ("fence.i" ::: "memory");
 }
 
 void invalidate_dcache_range(unsigned long start, unsigned long end)