pinctrl: ralink: rt305x: add missing include

Header 'rt305x.h' is ralink architecture dependent file where
other general definitions which are in 'ralink_regs.h' are
being used. This 'rt305x.h' is only being included in two
different files: 'rt305x.c' and 'pinctrl-rt305x.c'. When
file 'pinctrl-rt305x.c' is being compiled definitions in
'ralink_regs.h' are need to build it properly. Hence, add
missing include 'ralink_regs.h' in 'pinctrl-rt305x.c'
source to avoid compilation problems.

Fixes: 3a1b0ca5

 ("pinctrl: ralink: move RT305X SoC pinmux config into a new 'pinctrl-rt305x.c' file")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210629143407.14703-1-sergio.paracuellos@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>