Update BeagleBoard defconfig:

- Use MPU timer instead of 32k timer to work around serial hang issue
- Disable CONFIG_DEBUG_LL
Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Acked-by: Syed Mohammed Khasim <khasim@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>