n1sdp: add delay between link training and bus enumeration

This patch adds delay before bus enumeration after link training to
cater devices connected in downstream ports in PCIe switch to
finish their link training completely.

Change-Id: I2011dcd8fdd87e4a74d4a651ecaf75ab970a7ae3
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>