boards/nucleo_f103rb: enable 72MHz system clock by default

Enable 72MHz SYSCLK by default. The board does not have an on-board
quartz, however the STLink frontend produces a 8MHz clock signal that we
can use. Since the clock signal is not coming from an oscillator, HSE
bypass must be enabled. Make sure not to exceed 36MHz clock on APB1 bus.

Change-Id: I6b0b499a1cc4b0deccbfa374fc9ca3e3e8cc38c5
Signed-off-by: Maciej Borzecki <maciek.borzecki@gmail.com>