ARM: increase lmb stack space reservation to 4KB

The bootm initrd image copy to ram can collide with the stack in cases
where the print buffer size is large (i.e. 1K). The result is intermittent
initrd decompression errors depending on the initrd size MOD 4KB since
the initrd start address is 4KB aligned.

Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>