malloc: dlmalloc: add an ability for the malloc to be re-init/init multiple times

Malloc gets initialized with a call to mem_malloc_init() with the address
the allocation starts to and its size. Currently it is not possible to
move the malloc from one memory area to another as the malloc would eventually
fail.

This patch adds in the ability to re-init the malloc with the updated
start address and the size.

One of the use cases of this feature is SPL U-Boot running from within
the static memory and calling to malloc init from within board_init_f():

	arch/arm/cpu/armv8/start.S:reset vector
	arch/arm/cpu/armv8/start.S:main()
	arch/arm/lib/crt0_64.S:board_init_f()
	board/<my_board>/common/spl.c:board_init_f()
        board/<my_board>/common/spl.c:mem_malloc_init((ulong)CONFIG_SYS_SPL_MALLOC_START,
                        CONFIG_SYS_SPL_MALLOC_SIZE);

Shortly after the DDR (main) memory is init and ready we call to malloc init
again but this time with the start address in the DDR memory and a much greater
size for moving the allocation off the static to the DDR memory:

	board/<my_board>/common/spl.c:mem_malloc_init((ulong)CONFIG_SPL_MALLOC_OFFSET,
			CONFIG_SPL_MALLOC_SIZE);

Where CONFIG_SYS_SPL_MALLOC_START and CONFIG_SPL_MALLOC_OFFSET are the start
addresses of the malloc in the static and DDR memories respectively and
CONFIG_SYS_SPL_MALLOC_SIZE=SZ_16K and CONFIG_SPL_MALLOC_SIZE=SZ_2M are
the sizes of the mallocs in these memories. Note, now we have a much greater
memory, enlarging from 16K to 2M, available for allocation.

There is an alternative approach already existing in U-Boot with the use of
an early (simplified) malloc and the proper (dlamalloc) malloc however
necessitating managing the two mallocs whereas this approach proposes using
a single dlmalloc.

Signed-off-by: Marek Bykowski <marek.bykowski@gmail.com>