rockchip: rk3399-puma: add boot-on regulator to override BIOS_DISABLE

The (Qseven) BIOS_DISABLE signal on the RK3399-Q7 (Puma) keeps the
eMMC and SPI in reset initially and we need to write a GPIO to turn
them on before continuing the boot-up.

This adds the DTS entries for the additional regulator and makes
pinctrl and gpio3 available during SPL.  It also adds a hook to the
spl_board_init() to ensure that the regulator gets probed and enabled.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>