docs/pinctrl: fix the reference to the u300 platform

With commit ce1380c ("ARM: remove u300 platform") it is wrong to
use arch/arm/mach-u300/Kconfig file as example. Since the u300 platform
has been replaced by the u8500, let's use its Kconfig as example.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Link: https://lore.kernel.org/r/20210527201309.13308-1-dariobin@libero.it
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/pin-control.rst

@@ -95,7 +95,7 @@ this in our driver::
 To enable the pinctrl subsystem and the subgroups for PINMUX and PINCONF and
 selected drivers, you need to select them from your machine's Kconfig entry,
 since these are so tightly integrated with the machines they are used on.
-See for example arch/arm/mach-u300/Kconfig for an example.
+See for example arch/arm/mach-ux500/Kconfig for an example.
 
 Pins usually have fancier names than this. You can find these in the datasheet
 for your chip. Notice that the core pinctrl.h file provides a fancy macro