Increase default of CONFIG_SYS_MALLOC_F_LEN for SPL_OF_CONTROL

If both SPL_DM and SPL_OF_CONTROL are enabled, SPL needs to bind
several devices, but CONFIG_SYS_MALLOC_F_LEN=0x400 is apparently
not enough.  Increase the default to 0x2000 for the case.  This
will be helpful for shorter defconfigs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>