arm: apple: Remove CONFIG_SYS_SDRAM_BASE

The memory layout is taken from the device tree passed to us by
m1n1, so there is no need to define this.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>