malloc_simple: Add support for switching to DRAM heap

malloc_simple uses a part of the stack as heap, initially it uses
SYS_MALLOC_F_LEN bytes which typically is quite small as the initial
stacks sits in SRAM and we do not have that much SRAM to work with.

When DRAM becomes available we may switch the stack from SRAM to DRAM
to give use more room. This commit adds support for also switching to
a new bigger malloc_simple heap located in the new stack.

Note that this requires spl_init to be called before spl_relocate_stack_gd
which in practice means that spl_init must be called from board_init_f.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>