sunxi: move early "SRAM setup" into separate file

Currently we do some magic "SRAM setup" MMIO writes in s_init(), copied
from the original BSP U-Boot. The comment speaks of this being required
before DRAM access gets enabled, but there is no indication that this
would actually be required that early.

Move this out of s_init(), into board_init_f(). Since this actually only
affects a very few older SoCs, the actual code goes into the cpu/armv7
directory, to move it out of the way for all other SoCs.

This also uses the opportunity to convert some #ifdefs over to the fancy
IS_ENABLED() macros used in actual C code.

We keep the s_init() stub around for now, since armv8's lowlevel_init
still relies on it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>