Convert CONFIG_TEGRA_GPU to Kconfig

This converts the following to Kconfig:
   CONFIG_TEGRA_GPU

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/nyan-big_defconfig b/configs/nyan-big_defconfig
index 489ff63..a48920c 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DEBUG_UART_CLOCK=408000000
 CONFIG_TEGRA124=y
 CONFIG_TARGET_NYAN_BIG=y
+CONFIG_TEGRA_GPU=y
 CONFIG_SYS_LOAD_ADDR=0x82408000
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y