arm64: dts: vexpress: Fix VExpress LED names

The common LED binding wants the LED node names to start with led- and
then have just a single number.

Changing the naming for the 8 user LEDs from using user<x> to led-<x>.
Also there is no default-trigger named "mmc0" in the kernel, so use the
more generic "disk-activity".

Link: https://lore.kernel.org/r/20200513103016.130417-18-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>