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>
Acked-by: Tom Rini <trini@ti.com>