ARM: dts: sh73a0: Rename twd clock to periph clock

The "TWD" clock is actually the Cortex-A9 MPCore "PERIPHCLK" clock,
which not only clocks the private timers and watchdogs (TWD), but also
the interrupt controller and global timer.

Hence rename it from "twd" to "periph".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191211135222.26770-2-geert+renesas@glider.be