drivers: (le)u(s)art_gecko: use DT defined clock identifiers

Use the device tree to assign the correct peripheral clock to each
UART/USART/LEUART. Previously, the clock identifier was determined
through the sequence number of the instantiated UART. This meant
configuring all UARTs when only one of the later UARTs was required.

Signed-off-by: Oane Kingma <o.kingma@interay.com>