PCI: mvebu: Fix support for PCI_EXP_RTSTA on emulated bridge

PCI: mvebu: Fix support for PCI_EXP_RTSTA on emulated bridge

BugLink: https://bugs.launchpad.net/bugs/1964361
[ Upstream commit 838ff44a398ff47fe9b924961d91aee325821220 ]

PME Status bit in Root Status Register (PCIE_RC_RTSTA_OFF) is read-only and
can be cleared only by writing 0b to the Interrupt Cause RW0C register
(PCIE_INT_CAUSE_OFF).

Link: https://lore.kernel.org/r/20211125124605.25915-15-pali@kernel.org
Fixes: 1f08673eef12 ("PCI: mvebu: Convert to PCI emulated bridge config space")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>