[clkmgr] Fix core name
[clkmgr] Fix core name

The clkmgr_pkg core file could overwrite the rstmgr_pkg core file
depending on how fusesoc happens to pick up files, leading to a broken
build.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan