docs/pinctrl: fix the reference to the u300 platform

With commit ce1380c9

 ("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has been replaced by the u8500, let's use its Kconfig as example.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>