u-boot-qoriq: add 'u-boot' to RPROVIDES

the name of u-boot recipe has been changed in latest up-stream master of
meta-fsl-ppl, so we need to explicitly add 'u-boot' to its RPROVIDES as the
image has a runtime dependency on u-boot

http://jira.alm.mentorg.com:8080/browse/SB-3426

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>