Initialise rpi-firmware before clk-bcm2835

The IMA (Integrity Measurement Architecture) looks for a TPM (Trusted
Platform Module) having been registered when it initialises; otherwise
it assumes there is no TPM. It has been observed on BCM2835 that IMA
is initialised before TPM, and that initialising the BCM2835 clock
driver before the firmware driver has the effect of reversing this
order.

Change the firmware driver to initialise at core_initcall, delaying the
BCM2835 clock driver to postcore_initcall.

See: https://github.com/raspberrypi/linux/issues/3291
     https://github.com/raspberrypi/linux/pull/3297



Signed-off-by: Luke Hinds <lhinds@redhat.com>
Co-authored-by: Phil Elwell <phil@raspberrypi.org>