DT: rp1: add general-purpose clock source definitions

GPCLKs have two parts - a clock divider and a clock input, routed out to
and in from a GPIO pad respectively. It follows that the clksrc_gpN
inputs can't be used unless the pad is also configured as a GPCLK, so
leave them disabled.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>