sunxi: H616: dram: const-ify DRAM function parameters

There are quite some functions in the Allwinner H616 DRAM "driver", some
of them actually change the parameters in the structure passed to them,
but many are actually not.
To increase the optimisation potential for the code, mark those functions
that just read members of the passed dram_para struct as "const".
This in itself does not decrease the code size, but lays the groundwork
for future changes doing so.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>