sunxi: Move SPL s_init() code to board_init_f()

The current sunxi implementation uses gdata, which is going away. It also
sets up DRAM before board_init_f() in SPL.

There is really no reason to do much in s_init() since board_init_f() is
called immediately afterwards. The only change is that we need our own
implementation of board_init_f() which sets up DRAM before the BSS (which
is in DRAM) is cleared.

The s_init() code runs once for SPL and again for U-Boot proper. We
shouldn't need to init the clock/timer/gpio/i2c init twice, so just have it
in SPL.

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