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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 16b373d..e0bbabd 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -8,6 +8,7 @@
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_OFFSET=0x80000
 CONFIG_SYS_LOAD_ADDR=0x23000000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20"
 CONFIG_BOOTDELAY=3