firmware: raspberrypi: Change dependency to ARCH_BCM2835 and COMPILE_TEST

firmware: raspberrypi: Change dependency to ARCH_BCM2835 and COMPILE_TEST

The Raspberry Pi firmware driver has no compile dependencies on the
BCM2835 mailbox driver. It's just a indirect runtime dependency: the
driver only works on a Raspberry Pi.

Change the dependency from BCM2835_MBOX to ARCH_BCM2835. Also allow
compile tests. This allows drivers that have build time dependencies
on this firmware driver to be compile tested as well. More dependencies
are added to account for build time dependencies:

  - depends on MAILBOX for mailbox API (not stubbed) usage
  - depends on ARM || ARM64 for dsb() usage

Also make it built by default if ARCH_BCM2835, since many functions will
not work without this firmware driver.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20260427091058.2669812-1-wenst@chromium.org
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>