power: pmic: sunxi: only build AXP drivers for SPL

power: pmic: sunxi: only build AXP drivers for SPL

The axp<xxx>.c drivers are only used for the SPL, for U-Boot proper we
have a separate, DM compliant driver.
Mask the build instructions with CONFIG_SPL_BUILD, to avoid them being
build for U-Boot proper as well.

The AXP221 driver defines axp_get_sid(), which is used in the U-Boot
proper cpuinfo() code, and some old LCD code directly calls axp_set_eldo(),
so we keep that driver outside the new guards. This will be fixed properly
later.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>