Add definitions for the KEEPON_* bits in the "BKUP Mode Cnt" register,
which control the DDR rails to be kept powered when backup mode is
enabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>