malloc_f: enable SYS_MALLOC_F by default if DM is on

This option has a bool type, not hex.
Fix it and enable it if CONFIG_DM is on because Driver Model always
requires malloc memory.  Devices are scanned twice, before/after
relocation.  CONFIG_SYS_MALLOC_F should be enabled to use malloc
memory before relocation.  As it is board-independent, handle it
globally.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Robert Baldyga <r.baldyga@samsung.com>