doc: relnotes: Add SPI release notes

Not much activity, but a few changes to remark on the relnotes.

Closes #46218.

Signed-off-by: Carles Cufi <[email protected]>

Author: carlescufi

doc/releases/release-notes-3.1.rst

@@ -542,6 +542,9 @@ Drivers and Sensors
 
 * SPI
 
+  * Ported all SPI drivers to pinctrl
+  * Added support for SPI on the GD32 family
+
 * Timer
 
 * USB