microblaze: Change temp register for cmdline

For copy was used r7 register when CONFIG_CMDLINE_BOOL option
is enabled. But r7 stores pointer to fdt that's why machine_early_init
not detect compiled-in DTB.

I also moved kernel PID setup to have TLB init in one block

Signed-off-by: Michal Simek <monstr@monstr.eu>