module/pcie-integ-ctrl: Conditionally subscribe to clock notifications

If on a platform, power domains and clock domains required by this module
are enabled before the start, there is no need to subscribe for the
clock notifications. Therefore subscribe to clock notifications only if
clock_id is valid in the configuration.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>
Change-Id: I5048b20fa2c8a0e8d96d938dde47d375b310dd00