clk: socfpga: fix clock driver for 3.15

commit [1771b10d

 clk: respect the clock dependencies in of_clk_init]
exposed a flaw in the socfpga clock driver and prevents the platform
from booting on 3.15-rc1.

Because the "altr,clk-mgr" is not really a clock, it should not be using
CLK_OF_DECLARE, instead we should be mapping the clk-mgr's base address
one of the functional clock init function. Use the socfpga_pll_init function
to map the clk_mgr_base_addr as this clock should always be initialized first.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Tested-by: Pavel Machek <pavel@denx.de>