Network driver updates.

• Move RMI MIPS extension to atomic increment word (LDADDWU) to common header file sys/mips/rmi/rmi_mips_exts.h
• Fix xlr_ldaddwu() for 64 bit, it is a 32 bit operation, use unsigned int* instead of unsigned long* argument
• Provide dummy xlr_enable_kx/xlr_restore_kx for n32 and n64.
• Provide xlr_paddr_ld() instead of xlr_paddr_lw(), so that the descriptor formats are same for 32 and 64 bit
• update nlge and rge for the changes

These changes are also needed by the security driver which will be
added later.