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/orangepi_zero2_defconfig b/configs/orangepi_zero2_defconfig
index d9c89c8..6211754 100644
--- a/configs/orangepi_zero2_defconfig
+++ b/configs/orangepi_zero2_defconfig
@@ -11,6 +11,7 @@
 CONFIG_R_I2C_ENABLE=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_MAX_SIZE=0xbfa0
+CONFIG_SPL_STACK=0x58000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SPL_SYS_I2C_LEGACY=y