PCI: thunder-pem: Fix legacy firmware PEM-specific resources

SZ_16M PEM resource size includes PEM-specific register and its children
resources. Reservation of the whole SZ_16M range leads to child device
driver failure when pcieport driver is requesting resources:

  pcieport 0004:1f:00.0: can't enable device: BAR 0 [mem 0x87e0c0f00000-0x87e0c0ffffff 64bit] not claimed

So we cannot reserve full 16M here and instead we want to reserve
PEM-specific register only which is SZ_64K.

At the end increase PEM resource to SZ_16M since this is what
thunder_pem_init() call expects for proper initialization.

Fixes: 9abb27c7

 ("PCI: thunder-pem: Add legacy firmware support for Cavium ThunderX host controller")
Signed-off-by: Tomasz Nowicki <tn@semihalf.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v4.10+