ARM: dts: uniphier: add reference clock nodes

Add master clock nodes generated by crystal oscillators.

  PH1-sLD3, PH1-LD4: 24.576 MHz
  PH1-Pro4, ProXstream2: 25.000 MHz
  PH1-Pro5: 20.000 MHz

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