mb/emulation/spike-riscv/uart.c: Update UART address

Spike Simulator commit 191634d2854d implemented a ns16550 serial device
which puts the base address at 0x10000000.

Tested: Start Spike Simulator and see that coreboot prints onto the UART.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I0e3db9d8b141c733bf609f906018096e3594ce83
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85852
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/emulation/spike-riscv/uart.c

@@ -5,5 +5,5 @@
 
 uintptr_t uart_platform_base(unsigned int idx)
 {
-	return (uintptr_t)0x02100000;
+	return (uintptr_t)0x10000000;
 }