rockchip: Avoid #ifdefs in RK3399 SPL

The code here is confusing due to large blocks which are #ifdefed out.
Add a function phase_sdram_init() which returns whether SDRAM init
should happen in the current phase, using that as needed to control the
code flow.

This increases code size by about 500 bytes in SPL when the cache is on,
since it must call the rather large rockchip_sdram_size() function.

Signed-off-by: Simon Glass <sjg@chromium.org>