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/gazerbeam_defconfig b/configs/gazerbeam_defconfig
index 23d10bf..91f9134 100644
--- a/configs/gazerbeam_defconfig
+++ b/configs/gazerbeam_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_MEMTEST_END=0x07e00000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_SECT_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x80000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="gazerbeam"
 CONFIG_IDENT_STRING=" gazerbeam 0.01"