Add back fake us delay.

Author: tannewt

ports/litex/mphalport.c

@@ -40,6 +40,10 @@
     void hal_dcd_isr(uint8_t rhport);
 #endif
 
+void mp_hal_delay_us(mp_uint_t delay) {
+    mp_hal_delay_ms(delay / 1000);
+}
+
 extern void SysTick_Handler(void);
 
 __attribute__((section(".ramtext")))