serial: lpuart: Enable IPG clock

Current codes only ennable the PER clock. However on iMX8 the LPUART
also needs IPG clock which is an LPCG. Should not depend on the default
LPCG setting.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>