posix: add missing delay_us and delay_ms functions

src/arch/posix/arch.cc

 #include "arch.h"
+#include <time.h>
 #include <unistd.h>
 
 #if defined(WITH_LOOP) || defined(TIMER_S)
@@ -31,4 +32,20 @@ void Arch::idle(void)
 {
 }
 
+void Arch::delay_us(unsigned int const us)
+{
+	struct timespec ts;
+	ts.tv_sec = 0;
+	ts.tv_nsec = us * 1000;
+	nanosleep(&ts, NULL);
+}
+
+void Arch::delay_ms(unsigned int const ms)
+{
+	struct timespec ts;
+	ts.tv_sec = ms / 1000;
+	ts.tv_nsec = (ms % 1000) * 1000000;
+	nanosleep(&ts, NULL);
+}
+
 Arch arch;