arm926ejs: timer: Replace bss variable by gdr

Reuse the gd->tbl value for timestamp and add gd->lastinc for lastinc bss
values in the arm926ejs timers implementation.

The usage of bss values in drivers before initialisation of bss is forbidden.
In that special case some data in .rel.dyn gets corrupted.

This patch is similiar to the patch Dirk Behme posted
for the armv7/omap-common/timer.c and added suggestions
from Reinhard Meyer.

Tested on the arm926ejs mx27 based magnesium board
Tested on the arm926ejs kirkwood based suen3 board

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Albert ARIBAUD <albert.aribaud@free.fr>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Stefano Babic <sbabic@denx.de>
cc: Reinhard Meyer <u-boot@emk-elektronik.de>