boards: x86: Increase minimum system heap size for ACPI

ACPICA initialization takes considerably more than 32k. E.g. on
up_squared the utilization is 400-500k, qemu_x86_64 about 120k and on
EHL about 1.5M. Increase the default on these platforms to be big enough
if ACPI is enabled.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>