arm: Set up global data before board_init_f()

At present arm defines CONFIG_SYS_GENERIC_GLOBAL_DATA, meaning that
the global_data pointer is set up in board_init_f(). However it is
actually set up before this, it just isn't zeroed.

If we zero the global data before calling board_init_f() then we
don't need to define CONFIG_SYS_GENERIC_GLOBAL_DATA.

Make this change (on arm32 only) to simplify the init process. I
don't have the ability to test aarch64 yet.

Signed-off-by: Simon Glass <sjg@chromium.org>
Merge branch 'v2014.10_patch_armv4t' into v2014.10_merged
Merge branch 'v2014.10_patch_clang' into v2014.10_merged
Merge branch 'v2014.10_patch_config' into v2014.10_merged
Merge branch 'v2014.10_patch_init_gd' into v2014.10_merged
Merge branch 'v2014.10_patch_tam3517' into v2014.10_merged
Merge branch 'v2014.10_patch_wrn_W1_arm_read' into v2014.10_merged
Merge branch 'v2014.10_patch_wrn_ipu_disp' into v2014.10_merged
Merge branch 'v2014.10_patch_wrn_lcd_puts_xy' into v2014.10_merged