nios2: pass command line and initrd to linux in bootm.c

This patch adds bootargs passing to nios2 linux.

The args passing is enabled with,
r4 : 'NIOS' magic
r5 : pointer to initrd start
r6 : pointer to initrd end
r7 : pointer to command line

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>