clk: bcm2835: Drop the fixed sys_pclk.

Nothing uses it, and I can't find any evidence that anything ever has.
Its role is now filled by the core clock in the firmware driver.

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