rockchip: rk3188: use boot0 hook to load up SPL in 2 steps

rockchip: rk3188: use boot0 hook to load up SPL in 2 steps

For the RK3188, the BROM will attempt to load up the first stage
image (SPL for the RK3188) in two steps: first 1KB to offset 0x800
in the SRAM and then the remainder to offset 0xc00 in the SRAM.
It always enters at 0x804, though.

With this changeset, the RK3188 boot removes the TPL (stub) stage and
builds a single SPL binary that utilizes the early back-to-bootrom via
the boot0-hook.

Consequently, the passing of the saved boot params via pmu->os_reg[2]
is also removed.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>