ARM: bcm2835: remove useless variables from Makefile.boot

Neither params_phys-y nor initrd_phys-y are needed when booting with a
Device Tree. Those values are passed through the Device Tree blob. So
get rid of those variable definitions from Makefile.boot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>