arm64: dts: uniphier: fix I2C nodes of PH1-LD20

The I2C hardware blocks on this SoC are connected as follows:

  I2C0: external connection
  I2C1: external connection
  I2C2: internal connection
  I2C3: external connection
  I2C4: external connection
  I2C5: internal connection
  I2C6: no connection (not accessible)

Delete pinctrl from Ch2, add pinctrl to Ch4, and remove the Ch6 node.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>