clk: bcm2835: Mark the VPU clock as critical

The VPU clock is also the clock for our AXI bus, so we really can't
disable it.  This might have happened during boot if, for example,
uart1 (aux_uart clock) probed and was then disabled before the other
consumers of the VPU clock had probed.

v2: Rewrite to use a .flags in bcm2835_clock_data, since other clocks
    will need this too.

Signed-off-by: Eric Anholt <eric@anholt.net>