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/zynq_cse_qspi_defconfig b/configs/zynq_cse_qspi_defconfig
index 2f32f9d..241bf0c 100644
--- a/configs/zynq_cse_qspi_defconfig
+++ b/configs/zynq_cse_qspi_defconfig
@@ -30,6 +30,8 @@
 CONFIG_CLOCKS=y
 CONFIG_SPL_MAX_SIZE=0x30000
 CONFIG_SPL_BSS_MAX_SIZE=0x8000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xfffffe00
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x100000