PCI: endpoint: pci-epf-test: Allow overriding default BAR sizes

PCI: endpoint: pci-epf-test: Allow overriding default BAR sizes

Add bar{0,1,2,3,4,5}_size attributes in configfs, so that the user is not
restricted to run pci-epf-test with the hardcoded BAR size values defined
in pci-epf-test.c.

This code is shamelessly more or less copy pasted from pci-epf-vntb.c

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Koichiro Den <den@valinux.co.jp>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260130113038.2143947-2-cassel@kernel.org