ASRock E350M1: mainboard.c: Add declarations for `set_pcie_{,de}reset`

Since the merg of the ASRock E350M1 port (a649a96e) the compiler
warns about the following [1].

    mainboard.c:35, GNU Compiler 4 (gcc), Priorität: Normal
    no previous prototype for 'set_pcie_reset' [-Wmissing-prototypes]
    mainboard.c:43, GNU Compiler 4 (gcc), Priorität: Normal
    no previous prototype for 'set_pcie_dereset' [-Wmissing-prototypes]

Adding the function prototypes to the beginning of the file as
done in commit »Persimmon updates for AMD F14 rev C0« (d7a696d0)
addresses the warning.

[1] http://qa.coreboot.org/job/coreboot-gerrit/4975/warnings13Result/package.-139448264/file.-1544928473/
[2] http://review.coreboot.org/137

Change-Id: Iad2e62ec37c3a2f749a264974b61ac7c226e9b83
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/2590
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/asrock/e350m1/mainboard.c

@@ -27,6 +27,8 @@
 //#include <southbridge/amd/sb800/sb800.h>
 
 //#define SMBUS_IO_BASE 0x6000
+void set_pcie_reset(void);
+void set_pcie_dereset(void);
 
 /**
  * TODO