pinctrl: uniphier: remove obsoleted compatibles

Since commit 3e030b0b ("pinctrl: uniphier: allow to have pinctrl
node under syscon node"), this driver has kept compatibility for the
old DT files.  Several releases have passed since then, so remove
the obsoleted compatibles and clean up the code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>