ARM64: dts: meson-gx: fix UART pclk clock name

The clock-names for pclk was wrongly set to "core", but the bindings
specifies "pclk".
This was not cathed until the legacy non-documented bindings were removed.

Reported-by: Andreas Färber <afaerber@suse.de>
Fixes: f72d6f60

 ("ARM64: dts: meson-gx: use stable UART bindings with correct gate clock")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>