mvebu: Use CONFIG_SPL_STACK + 4 directly for bootparam location

mvebu: Use CONFIG_SPL_STACK + 4 directly for bootparam location

The definition of CONFIG_SPL_BOOTROM_SAVE is always a fixed
CONFIG_SPL_STACK + 4, while CONFIG_SPL_STACK is not constant.  This
change will make it clear where the location is still, once
CONFIG_SPL_STACK moves to Kconfig.

Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>