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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index a7698ac..86bda86 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -28,6 +28,8 @@
 CONFIG_SPL_MAX_FOOTPRINT=0xffb00
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x100000
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_OS_BASE=0x2c060000