boards: efm32gg_sltb009a: uart: Fix USART0 loc 1

Silabs Thunderboard EFM32GG12 (OPN: SLTB009A) connects USART0
location 1 to the VCOM pins of the on-board debugger.

US0_TX loc 1 is pin PE7
US0_RX loc 1 is pin PE6

Signed-off-by: Alex Hogen <alex@edt.com>
1 file changed