mips: unify prom_putchar() declarations

prom_putchar() is used centrally in early printk infrastructure therefore
at least MIPS should agree on the function return type.

[paul.burton@mips.com:
  - Include linux/types.h in asm/setup.h to gain the bool typedef before
    we start include asm/setup.h elsewhere.
  - Include asm/setup.h in all files that use or define prom_putchar().
  - Also standardise on signed rather than unsigned char argument.]

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19842/
Cc: linux-mips@linux-mips.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>

arch/mips/paravirt/serial.c

@@ -9,16 +9,15 @@
 #include <linux/kernel.h>
 #include <linux/virtio_console.h>
 #include <linux/kvm_para.h>
+#include <asm/setup.h>
 
 /*
  * Emit one character to the boot console.
  */
-int prom_putchar(char c)
+void prom_putchar(char c)
 {
 	kvm_hypercall3(KVM_HC_MIPS_CONSOLE_OUTPUT, 0 /*  port 0 */,
 		(unsigned long)&c, 1 /* len == 1 */);
-
-	return 1;
 }
 
 #ifdef CONFIG_VIRTIO_CONSOLE