juno: CDCEL clock driver uses incorrect address for I2C transaction

The first four bytes of the PLL Config register are not used by Juno, so
we always skip them when reading/writing the PLL configuration. The
address was not incremented for the write after a read when setting the
PLL data in set_rate_from_index() call.

Change-Id: I094d3648168a1142cba4ee90e51aa31b3b93de88
Signed-off-by: Jim Quigley <jim.quigley@arm.com>