lib:platform: Fix sbi_getc return type.

As per the current SBI specification, sbi_getc should return
an int instead of char.

In case of FIFO is empty, return -1 as per the specification.

Reported-by: Sergi Granell <xerpi.g.12@gmail.com>
Suggested-by:Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
Signed-off-by: Atish Patra <atish.patra@wdc.com>

platform/template/platform.c

@@ -73,7 +73,7 @@ static void platform_console_putc(char ch)
 /*
  * Read a character from the platform console input.
  */
-static char platform_console_getc(void)
+static int platform_console_getc(void)
 {
 	return uart8250_getc();
 }