stm32/boards/NUCLEO_F429ZI: Incr CPU freq to 168MHz to get USB working.

At the original frequency of 90MHz there's no way to get a 48MHz USB clock.
These new setting mirror those of the STM32F429DISC board.

ports/stm32/boards/NUCLEO_F429ZI/mpconfigboard.h

@@ -16,9 +16,9 @@
 
 // HSE is 8MHz
 #define MICROPY_HW_CLK_PLLM (8)
-#define MICROPY_HW_CLK_PLLN (180)
+#define MICROPY_HW_CLK_PLLN (336)
 #define MICROPY_HW_CLK_PLLP (RCC_PLLP_DIV2)
-#define MICROPY_HW_CLK_PLLQ (4)
+#define MICROPY_HW_CLK_PLLQ (7)
 
 // From the reference manual, for 2.7V to 3.6V
 // 151-180 MHz => 5 wait states