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/alchemy/board-gpr.c

@@ -34,6 +34,7 @@
 #include <asm/bootinfo.h>
 #include <asm/idle.h>
 #include <asm/reboot.h>
+#include <asm/setup.h>
 #include <asm/mach-au1x00/au1000.h>
 #include <asm/mach-au1x00/gpio-au1000.h>
 #include <prom.h>
@@ -60,7 +61,7 @@ void __init prom_init(void)
 	add_memory_region(0, memsize, BOOT_MEM_RAM);
 }
 
-void prom_putchar(unsigned char c)
+void prom_putchar(char c)
 {
 	alchemy_uart_putchar(AU1000_UART0_PHYS_ADDR, c);
 }