ARM: bcm2835: Move to mach-bcm directory

Move the bcm2835 board file into the mach-bcm directory. This allows us
to get rid of the mach-bcm2835 directory with the associated Kconfig
and Makefile.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
[swarren, adjust defconfig so ARCH_BCM2835 still gets enabled]
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>