clk: socfpga: Look for the GPIO_DB_CLK by its offset

After the patch:
"clk: socfpga: Map the clk manager base address in the clock driver"

The clk->name field in socfpga_clk_recalc_rate() was getting cleared. Replace
looking for the GPIO_DB_CLK by its divider offset instead.

Also rename the define SOCFPGA_DB_CLK_OFFSET -> SOCFPGA_GPIO_DB_CLK_OFFSET, as
this represents the GPIO_DB_CLK.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>