uboot-imx

mips: Add an 'include/asm/errno.h', like all other architectures

All other u-boot architectures have an include/asm/errno.h, so
this change adds it to the mips include/asm-mips headers also.

Stolen from Linux 2.6.25.

Signed-off-by: Jason McMullan <mcmullan@netapp.com>