Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/r7-tv-dongle_defconfig b/configs/r7-tv-dongle_defconfig
index a8452ba..f5adbd3 100644
--- a/configs/r7-tv-dongle_defconfig
+++ b/configs/r7-tv-dongle_defconfig
@@ -6,6 +6,7 @@
 CONFIG_DRAM_CLK=384
 CONFIG_USB1_VBUS_PIN="PG13"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_MVTWSI=y