Convert CONFIG_SYS_SPL_MALLOC_SIZE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPL_MALLOC_SIZE
   CONFIG_SYS_SPL_MALLOC_START

We introduce a default value here as well, and CONFIG_SYS_SPL_MALLOC to
control if we have a malloc pool or not.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx6ul_geam_nand_defconfig b/configs/imx6ul_geam_nand_defconfig
index 8bd4360..ac56f20 100644
--- a/configs/imx6ul_geam_nand_defconfig
+++ b/configs/imx6ul_geam_nand_defconfig
@@ -25,6 +25,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="run $modeboot"
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_DMA=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_WATCHDOG=y