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/apalis_imx6_defconfig b/configs/apalis_imx6_defconfig
index 11ee2f3..89e9971 100644
--- a/configs/apalis_imx6_defconfig
+++ b/configs/apalis_imx6_defconfig
@@ -36,6 +36,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_MISC_INIT_R=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_DMA=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_USB_HOST=y