bcm27xx/bcm2712: Fix-up RP1 modules

bcm27xx/bcm2712: Fix-up RP1 modules

Correct a few mistakes around dependencies and naming and unset
CONFIG_FIRMWARE_RP1 in RPi5B's config and instead of a builtin, build it
as a module.

Without this change, there are two entries for rp1.ko in
/lib/modules/$(uname -r)/modules.builtin due to how we strip the leading
directories when we generate it. See: package/kernel/linux/Makefile
around line 63.

  % grep rp1.ko /lib/modules/$(uname -r)/modules.builtin
  pwm-rp1.ko
  clk-rp1.ko
  rp1.ko
  rp1.ko

The kernel log gets spammed with tons of superfluous warnings as a
results of the double entry:

  daemon.warn modprobe: found duplicate builtin module rp1

Signed-off-by: John Audia <therealgraysky@proton.me>
Link: https://github.com/openwrt/openwrt/pull/17461
Signed-off-by: Robert Marko <robimarko@gmail.com>