imx: Use a convenient default value for SYS_MALLOC_F_LEN

imx: Use a convenient default value for SYS_MALLOC_F_LEN

Commit 3a7c45f6a772 ("simple-bus: add DM_FLAG_PRE_RELOC flag to
simple-bus driver") causes some i.MX boards that were converted
to DM, such as warp7, to fail to boot.

As explained by Lukas Auer:

"With the patch, U-Boot probes the drivers for devices under simple-bus
device tree nodes in the pre-relocation device model. The default value
of CONFIG_SYS_MALLOC_F_LEN (0x400) leaves U-Boot with not enough memory to
do this, causing it to hang."

Fix this problem by providing a convenient default value for
CONFIG_SYS_MALLOC_F_LEN.

Reported-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Suggested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Heiko Schocher <hs@denx.de>