soc: silabs: siwx91x: Rename UART instances

Currently, siwx917 have three instances of uart: ulpuart, uart1 and
uart2. However:

  - The other drivers on siwx91x (i2c, dma, i2s, etc...) rather use
    'ulp', '0' and '1'.

  - The reference manual also uses 'ulp', '0' and '1'.

The source of the confusion probably come from the clock driver in
WiseConnect which use clocks USART1 and USART2. However, this probably
not expected.

So, this patch renames uart1 and uart2 in uart0 and uart1. This change
also impacts the names of pins and the names of the clocks.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>