ARM: bcm2835: clarify RASPBERRYPI_FIRMWARE dependency

The firmware driver can be a loadable module, but the power domain
can only be built-in, so we get a build error in an allmodconfig
kernel:

:(.text+0x17e59c): undefined reference to `rpi_firmware_property'
:(.text+0x17e51c): undefined reference to `rpi_firmware_get'
:(.text+0x17e244): undefined reference to `rpi_firmware_property'

This changes the dependency to only allow the power domain code
to be enabled when the firmware driver is built-in. Other users
of the firmware driver may still be loadable modules and not
everyone needs the power domains, so we don't change the firmware
code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>