apple_m1_defconfig: Bump CONFIG_LMB_MAX_REGIONS to 64

Apple silicon SoCs have numerous embedded co-processors with pre-loaded
firmware. The co-processors text and data sections need to be mapped via
DART iommus controlled by the main processor. Those sections are
exported as reserved-memory. Bump CONFIG_LMB_MAX_REGIONS from 8 to 64 to
deal with the large amount of reserved-memory regions.

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>