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/gurnard_defconfig b/configs/gurnard_defconfig
index 183a6f9..0305105 100644
--- a/configs/gurnard_defconfig
+++ b/configs/gurnard_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_OFFSET=0x80000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g45-gurnard"
 CONFIG_SYS_LOAD_ADDR=0x23000000
 CONFIG_FIT=y