Convert CONFIG_SYS_MEM_TOP_HIDE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MEM_TOP_HIDE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/theadorable_debug_defconfig b/configs/theadorable_debug_defconfig
index 64ae291..230c086 100644
--- a/configs/theadorable_debug_defconfig
+++ b/configs/theadorable_debug_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SYS_LOAD_ADDR=0x800000
 CONFIG_DEBUG_UART=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_MEM_TOP_HIDE=0x80000
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y