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/socfpga_arria10_defconfig b/configs/socfpga_arria10_defconfig
index d326a3f..8b8d7e4 100644
--- a/configs/socfpga_arria10_defconfig
+++ b/configs/socfpga_arria10_defconfig
@@ -28,6 +28,8 @@
 CONFIG_CLOCKS=y
 CONFIG_SPL_PAD_TO=0x40000
 CONFIG_SPL_NO_BSS_LIMIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xffe2b000
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_FPGA=y
 CONFIG_SYS_MAXARGS=32