sunxi: Add DRAM_SUN8I_A23 kconfig entry

Add proper and simple kconfig option for dram_sun8i_a23.c
instead of using MACH type on Makefile.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>