clk: tegra: Mark APB clock as critical

clk: tegra: Mark APB clock as critical

Currently, the APB clock is registered with the CLK_IGNORE_UNUSED flag
to prevent the clock from being disabled if unused on boot. However,
even if it is used, it still needs to be always kept enabled so that it
doesn't get inadvertently disabled when all of its children are, and so
update the flag for the APB clock to be CLK_IS_CRITICAL.

Suggested-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>