board/mbx8xx/mbx8xx.c: Fix GCC 4.6 build warnings

board/mbx8xx/mbx8xx.c: Fix GCC 4.6 build warnings

Fix:
mbx8xx.c: In function 'mbx_init':
mbx8xx.c:127:15: warning: variable 'refclock' set but not used
[-Wunused-but-set-variable]
mbx8xx.c: In function 'board_get_cpufreq':
mbx8xx.c:231:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
mbx8xx.c: In function 'get_reffreq':
mbx8xx.c:242:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Wolfgang Denk <wd@denx.de>