ARM: dts: uniphier: fix IRQ number for devices on PH1-LD6b ref board

The IRQ signal from external devices on this board is connected to
the XIRQ4 pin of the SoC.  The IRQ number should be 52, not 50.

Fixes: a5e921b4

 ("ARM: dts: uniphier: add ProXstream2 and PH1-LD6b SoC/board support")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>