kirkwood: resize kernel partition for kirkwood devices

kirkwood: resize kernel partition for kirkwood devices

The 6.1 kernel has caused another increase in kernel size, and now
it's more than 3MB:

WARNING: Image file iom_ix4-200d-uImage is too big: 3170394 > 3145728
WARNING: Image file iom_ix2-200-uImage is too big: 3171494 > 3145728
WARNING: Image file linksys_e4200-v2-uImage is too big: 3171879 > 3145728
WARNING: Image file linksys_ea4500-uImage is too big: 3171871 > 3145728
WARNING: Image file linksys_ea3500-uImage is too big: 3171651 > 3145728

This causes problems for 5 devices:
  - Iomega StorCenter ix2/ix4
  - Linksys EA3500/EA4200/EA4500

They have enough resources for proper operation with 6.1, but all of
them had a 3MB kernel size limit. Let's keep them alive and
resize kernel partitions to 4MB.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>