ddr: altera: sdram: Make sdram_start and sdram_end into u32

Originally, both sdram_start and sdram_end were 64b values. The
sdram_start had no reason for being so, since our address space
is only 32b, so switching sdram_start to u32 is simple.

The sdram_end is a bit more complex, since it can actually be
set to (1 << 32) if someone really wanted to use an SoCFPGA with
4 GiB of DRAM and fixed the code around a little. But, the code
handling the protection rules internally decrements the sdram_end
variable anyway. Thus, instead of calling the code and passing in
the address of the SDRAM end, pass in the address already decremented
by one. This lets the sdram_end be 32b as well.

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