serial/serial_arc: set registers address during compilation

Being global variable with 0 value it falls into .bss area which we may
only use after relocation to RAM. And right afetr relocation we zero
.bss - effectively cleaing register address set for early console.

Now with pre-set value "regs" variable is no longer in .bss and this way
safely survives relocation.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>