sunxi: Enable Falcon boot mode parameters

this builds Falcon boot mode support. Not yet operational due to
spl_start_uboot() not yet implemented, so always boots u-boot.
sunxi: Move SPL stack to end of 32KB SRAM.
spl_mmc: Be a little more verbose when rejecting a loaded image
sunxi: Enable AXP Power chip version check again.

Only low 4 bits indicate chip version. Upper 4 bits is unknown.
power axp209: Function for checking poweron reason and powerbutton
sunxi: Enable Falcon boot control

This enables Falcon boot mode

to force normal u-boot mode power on by power button and immediately
press the power button again within 1/10 second.