hw/i386/pc: Add a description for the i8042 property

hw/i386/pc: Add a description for the i8042 property

While working on exposing the i8042 property in libvirt I noticed that
the property is missing a description. This adds a simple description
so that QEMU users don't have to dig in the source code to figure out
what this option does.

Signed-off-by: Kamil Szczęk <kamil@szczek.dev>
Link: https://lore.kernel.org/r/15fhXFY6x78KW8P5gw0eKTW8kc17zejrJFxqnOyoBy6vw4W9rCmgDhoxssWosWFs_dbFtfsyjn9wpPrV3x8Nlzhy8mTJSEnXCr4qyHAhXSw=@szczek.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>