doc: block diagram of HackRF

Source: http://2013.hackitoergosum.org/presentations/Day2-04.HackRF%20A%20Low%20Cost%20Software%20Defined%20Radio%20Platform%20by%20Benjamin%20Vernoux.pdf
Fix for "issues/62 fix PLL1 overclock bug" see hackrf_core.c -> cpu_clock_init()
Fix for "issues/78 startup current too high" see hackrf_core.c -> New functions cpu_clock_pll1_low_speed()/cpu_clock_pll1_max_speed() & hackrf_usb.c to switch low_speed/max_speed.
Merge branch 'master' of https://github.com/jboone/hackrf