xpl: Rename spl_phase() to xpl_phase()

Rename this function to indicate that it refers to any xPL phase.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/spl.rst b/doc/develop/spl.rst
index 4bb48e6..a92110e 100644
--- a/doc/develop/spl.rst
+++ b/doc/develop/spl.rst
@@ -34,6 +34,8 @@
            foo();
    #endif
 
+   if (xpl_phase() == PHASE_TPL)
+           bar();
 
 The building of SPL images can be enabled by CONFIG_SPL option in Kconfig.
 
@@ -117,7 +119,7 @@
 Checking the boot phase
 -----------------------
 
-Use `spl_phase()` to find the current U-Boot phase, e.g. `PHASE_SPL`. You can
+Use `xpl_phase()` to find the current U-Boot phase, e.g. `PHASE_SPL`. You can
 also find the previous and next phase and get the phase name.