toradex-smarc-imx8mp: add SPL_STACK size and SPL_HAVE_INIT_STACK

toradex-smarc-imx8mp: add SPL_STACK size and SPL_HAVE_INIT_STACK

When the toradex-smarc-imx8mp_defconfig file was first added, SPL_STACK
was set to 0x960000, but SPL_HAVE_INIT_STACK wasn't enabled.

This led to SPL_STACK being correctly dropped in commit 25fefa05d732
("configs: Resync with savedefconfig"), since SPL_HAVE_INIT_STACK was
missing, which ended up making the board not boot.

Fix this by adding SPL_STACK back and making sure SPL_HAVE_INIT_STACK is
enabled.

Fixes: dde53eae88d6 ("board: toradex: add Toradex SMARC iMX8MP")
Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com>