board_init: Do not reserve MALLOC_F area on stack if non-zero MALLOC_F_ADDR

In case the MALLOC_F_ADDR is set to non-zero value, the early malloc area is
not going to be placed just below stack top, but elsewhere. Do not reserve
MALLOC_F bytes in this case, as that wastes stack space and may even cause
insufficient stack space in SPL.

This functionality is particularly useful on i.MX8M, where the insufficient
stack space can be triggered.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Thomas Chou <thomas@wytron.com.tw>
Cc: Tom Rini <trini@konsulko.com>