power: regulator: rk8xx: Build get_ldo_reg only for SPL

Enabling CONFIG_SPL_POWER_SUPPORT will cause a compiler warning:
  ‘get_ldo_reg’ defined but not used [-Wunused-function]

Let's wrap get_ldo_reg(), rk808_ldo and rk818_ldo with ENABLE_DRIVER
which is only set for non SPL builds.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>