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/p2571_defconfig b/configs/p2571_defconfig
index b4e14ad..0315494 100644
--- a/configs/p2571_defconfig
+++ b/configs/p2571_defconfig
@@ -9,6 +9,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="tegra210-p2571"
 CONFIG_TEGRA210=y
 CONFIG_TARGET_P2571=y
+CONFIG_TEGRA_GPU=y
 CONFIG_SYS_LOAD_ADDR=0x80080000
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_CONSOLE_MUX=y