serial/uart_k20: Fix instance source clock frequencies

The uart_k20 serial driver incorrectly assumed that all instances of the
UART peripheral are driven by the system clock, when actually some
instances are driven by the bus clock (which usually runs at half the
system clock). This caused incorrect baud rate calculations for UART
instances driven by the bus clock (UART2-4).

Change-Id: I38041781cdee146912bb5167e7c71d6416b966b5
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>