esp32: Fix board cycle frequency

The ESP-WROOM-32 board exits the bootloader at 40 MHz, not 160 MHz as
suggested by documentation.  The CCOUNT special register works as
advertised, but not at the expected rate.  This was verified by
timestamping (at the host) the output of a dependency-free loop that
looks like:

	int key = irq_lock();
	while(1) {
		u32_t i, count;
		volatile int dummy;

		for(i = 0; i < 5000000; i++) {
			dummy++;
		}

		__asm__ volatile ("rsr.ccount %0" : "=a"(count));
		printk("%d\n", count);
	}

The SoC has a fairly robust set of possible CPU clocking modes, but we
don't have a driver for that yet.  Until we do, set the single
configured CPU frequency to the one we get at runtime.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>