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 BehĂșn <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/arch/arm/mach-mvebu/Kconfig b/arch/arm/mach-mvebu/Kconfig
index 54dff99..8365305 100644
--- a/arch/arm/mach-mvebu/Kconfig
+++ b/arch/arm/mach-mvebu/Kconfig
@@ -128,6 +128,7 @@
 	select I2C_MUX
 	select I2C_MUX_PCA954x
 	select SPL_I2C_MUX
+	select SPL_SYS_MALLOC_SIMPLE
 	select SYS_I2C_MVTWSI
 	select ATSHA204A