mx6: sys_proto: Add prototypes for imx6_pcie_toggle() functions

When compiling with W=1 errors are observed:

drivers/pci/pcie_imx.c:517:12: warning: no previous prototype for ‘imx6_pcie_toggle_power’ [-Wmissing-prototypes] __weak int imx6_pcie_toggle_power(void)

drivers/pci/pcie_imx.c:528:12: warning: no previous prototype for ‘imx6_pcie_toggle_reset’ [-Wmissing-prototypes] __weak int imx6_pcie_toggle_reset(void)

Remove these warnings by adding the functions prototypes on arch-mx6/sys_proto.

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>