Add a minimal FDT driver (bcm_gpclk) that consumes a BCM283x GP clock
via the clkdev framework. The driver is primarily intended as a test
harness for the existing CPRMAN clock driver: it exposes two sysctl
knobs under dev.bcm_gpclk.<unit>:
freq - read/write the GP clock frequency (Hz) via clk_set_freq(9) enable - gate/ungate the clock via clk_enable/clk_disable(9)
This makes it straightforward to exercise CPRMAN parent selection,
divider calculation, mux switching, and busy-wait logic from userland
without requiring any additional hardware beyond the SoC itself.
Add dpclk-rpi.dtso, a DTS overlay for BCM2711 (RPi 4) that routes
GPCLK0 to GPIO4 (ALT0) and instantiates a gpclk0 node consuming
BCM2835_CLOCK_GP0 from the CPRMAN clock provider.
Wire up the new files in files.bcm283x, files.arm64, and the rpi DTB
module Makefile.