common/board_f: Initialized global data for generic board

Some platforms (tested on mpc85xx, mpc86xx) use global data before calling
function baord_inti_f(). The data should not be cleared later. Any arch
which uses global data in generic board board_init_f() should define
CONFIG_SYS_GENERIC_GLOBAL_DATA.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Scott Wood <scottwood@freescale.com>
CC: Simon Glass <sjg@chromium.org>
CC: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Simon Glass <sjg@chromium.org>