arm: mvebu: turris_omnia: Move SPL's SYS_MALLOC_SIMPLE to Kconfig

Instead of declaring CONFIG_SYS_MALLOC_SIMPLE dependant on
CONFIG_SPL_BUILD in board config header, select
CONFIG_SPL_SYS_MALLOC_SIMPLE in Kconfig.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>