We can't just enable SYS_SUPPORTS_LITTLE_ENDIAN for successful
little-endian qemu-malta barebox build. Some byte swapping-related
macros are missed, e.g.:

    arch/mips/mach-malta/pci.c: In function 'pcibios_init':
    arch/mips/mach-malta/pci.c:218:28: error: 'GT_PCI0_CMD_MBYTESWAP_BIT'
    undeclared (first use in this function)
    GT_WRITE(GT_PCI0_CMD_OFS, GT_PCI0_CMD_MBYTESWAP_BIT |
                              ^

This patch adds necessary macros definition.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>