OMAP: disable gpmc timeout safely for reenabling

gpmc timeout is disabled and the reset counter
is set to 0. However, if later a driver activates
the timeout setting the reset to a valid value,
the old reset value with zero is still valid
for the first access. In fact, the timeout block
loads the reset counter after a successful access.

Found on a am335x board with a FPGA connected
to the GPMC bus together with the NAND.
When the FPGA driver in kernel activates
the timeout, the system hangs at the first access
by the NAND driver.

Signed-off-by: Stefano Babic <sbabic@denx.de>