mx5: Optimize lowlevel_init code size

Optimize mx5 lowlevel_init.S code size:
 - Compute values at compile time rather than at runtime where possible.
 - Assign r4 to hold the zero value rather than setting registers to 0 again and
   again.
 - Associate a function to setup_pll rather than expanding its large macro code
   multiple times.
 - Allocate constant values in section only if used.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Tested-by: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>