efm32: fix FTBFS on ARM due to alignment issues

The following warnings prevent OpenOCD from building:
efm32.c: In function 'efm32x_read_lock_data':
efm32.c:373:8: error: cast increases required alignment of target type [-Werror=cast-align]
efm32.c:386:9: error: cast increases required alignment of target type [-Werror=cast-align]
efm32.c:394:9: error: cast increases required alignment of target type [-Werror=cast-align]
efm32.c:402:9: error: cast increases required alignment of target type [-Werror=cast-align]
efm32.c: In function 'efm32x_get_page_lock':
efm32.c:430:17: error: cast increases required alignment of target type [-Werror=cast-align]
efm32.c: In function 'efm32x_set_page_lock':
efm32.c:441:19: error: cast increases required alignment of target type [-Werror=cast-align]
cc1: all warnings being treated as errors

This patch is compile-tested only.

Change-Id: Ia3a8f342e0f5e30c8ea4de9435c5c7a80bc100e3
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1370
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>