global: Rename SPL_ to XPL_

Use XPL_ as the symbol to indicate an SPL build. This means that SPL_ is
no-longer set.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mux/Makefile b/drivers/mux/Makefile
index d4e2478..63770e1 100644
--- a/drivers/mux/Makefile
+++ b/drivers/mux/Makefile
@@ -4,4 +4,4 @@
 # Jean-Jacques Hiblot <jjhiblot@ti.com>
 
 obj-$(CONFIG_MULTIPLEXER) += mux-uclass.o
-obj-$(CONFIG_$(SPL_)MUX_MMIO) += mmio.o
+obj-$(CONFIG_$(XPL_)MUX_MMIO) += mmio.o