arm64: dts: cavium: thunder2: Add missing PL011 "uartclk"

arm64: dts: cavium: thunder2: Add missing PL011 "uartclk"

The PL011 IP has 2 clock inputs for UART core/baud and APB bus. The
Thunder2 SoC is missing the core "uartclk". In this case, the Linux
driver uses single clock for both clock inputs. Let's assume that's how
the h/w is wired and make the DT reflect that.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250609215706.3009692-2-robh@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>