clk: socfpga: stratix10: use platform driver APIs

Use platform driver APIs to map memory so that it will automatically free
the memory in case of errors.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
[sboyd@kernel.org: Return -ENOMEM error pointers, check for error
pointer at call site]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>