arm: apple: Change SoC name from "m1" into "apple"

U-Boot is expected to support multiple generations of Apple SoCs
in a single binary with a single defconfig. Therefore it makes
more sense to set SYS_SOC to "apple".

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