arm: mvebu: a38x: serdes: Update comment about PCIE*_ENABLE_* defines

These are part of SOC_CONTROL_REG1 register, not PEX_CAPABILITIES_REG.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>