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/qemu_arm_defconfig b/configs/qemu_arm_defconfig
index 19508a7..ded58d3 100644
--- a/configs/qemu_arm_defconfig
+++ b/configs/qemu_arm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_SECT_SIZE=0x40000
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_TARGET_QEMU_ARM_32BIT=y
 CONFIG_ARMV7_LPAE=y
 CONFIG_AHCI=y