platform: recalculate heap size to support new tlb entry number

Previous patch introduced a change that using hart count as the default
number of tlb entries in the fifo. This makes the default tlb fifo size
grow in square with the number of harts. So the default heap size is
not enough to allocate tlb fifo when the hart count is big.

Fixes: 52fd64b8

 ("platform: Uses hart count as the default size of tlb info")
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>