ddr: altera: sdram: Clean up set_sdr_dram_timing*()

Get rid of the constant clrsetbits_le32(), instead prepare the whole
content of the register once and write it at the end of the function.
Merge set_sdr_dram_timing{1,2,3,4,lowpwr}() into single function
set_sdr_dram_timing() , since there's no point in keeping all this
stuff separate anymore.

Signed-off-by: Marek Vasut <marex@denx.de>