microblaze: Calculate kernel pad automatically

The kernel needs to setup the first two tlbs with pad
which is used for early page allocation which is used
by mapin_ram() to allocate tables for lowmem memory
before memory initialisation is done.
Calculate pad directly from lowmem size.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

diff --cc arch/microblaze/Kconfig
Simple merge

diff --cc arch/microblaze/kernel/head.S
Simple merge