Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index 5430004..48ec43b 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -7,6 +7,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x3e0000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x202000
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1028a-kontron-sl28"
 CONFIG_SPL_TEXT_BASE=0x18010000
 CONFIG_SYS_FSL_SDHC_CLK_DIV=1