boards/stm32_mimi_a15: enable 72MHz system clock by default

Enable 72MHz SYSCLK by default. We use the fact that there is an
on-board 8MHz quartz oscillator available as HSE clock signal. Make sure
not to exceed 36MHz clock limit on APB1.

Change-Id: I9ebc2144910253e68cd8a9b078884852f01c2cab
Signed-off-by: Maciej Borzecki <maciek.borzecki@gmail.com>