This will give us the ability to set the pixel and HDMI state machine
clocks for the VC4 KMS driver, change the CPU frequency, and
potentially gate clocks in the future (once we also write a power
domain driver).  It also gives the uart an explicit clock reference,
so that we don't need to change the physical addresses of the old
fixed clk_bcm2835.c clocks for Raspberry Pi 2 port.

Two clocks get their frequencies updated as a result of this.  One is
uart's apb_pclk, which was previously accidentally grabbing the fixed
uart0_pclk due to the apb_pclk not having clk_register_clkdev()
called.  The uart doesn't seem to do anything with apb_pclk other than
make sure it's on, so that appears safe (also, as far as I can see,
the apb clock is actually the same as the VPU clock).  The other is
EMMC, which according to the docs was supposed to be in the 50-100Mhz
range, but it turns out the firmware needed to change to running it at
the 250Mhz core clock speed to avoid a bug in clock domain crossing.

Additionally, anything using BCM2835_CLOCK_VPU will now have a correct
clock rate if the user configures the boot-time core clock speed using
config.txt.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>