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/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index 0980954..086b3ee 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -27,6 +27,7 @@
 CONFIG_BOOTCOMMAND="run default_boot"
 CONFIG_DEFAULT_FDT_FILE="ask"
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_FIT_IMAGE_TINY=y
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot-dtb.img"