ARM: dts: r8a7740: Add missing extal2 to CPG node

ARM: dts: r8a7740: Add missing extal2 to CPG node

The Clock Pulse Generator (CPG) device node lacks the extal2 clock.
This may lead to a failure registering the "r" clock, or to a wrong
parent for the "usb24s" clock, depending on MD_CK2 pin configuration and
boot loader CPG_USBCKCR register configuration.

This went unnoticed, as this does not affect the single upstream board
configuration, which relies on the first clock input only.

Fixes: b7ebadc81f64424a ("ARM: shmobile: r8a7740: add SoC clocks to DTS")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Link: https://lore.kernel.org/r/20200508095918.6061-1-geert+renesas@glider.be