nios2: enable malloc() pool before relocation

Enable malloc() pool before relocation, because this is needed
to use serial uclass.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>

configs/nios2-generic_defconfig

 CONFIG_NIOS2=y
-# CONFIG_SYS_MALLOC_F is not set
 CONFIG_TARGET_NIOS2_GENERIC=y
 CONFIG_DEFAULT_DEVICE_TREE="3c120_devboard"
 CONFIG_HUSH_PARSER=y