cfi: Add weak default function for flash_cmd_reset()

cfi: Add weak default function for flash_cmd_reset()

Currently the CFI driver issues both AMD and Intel reset commands.
This is because the driver doesn't know yet which chips are connected.
This dual reset seems to cause problems with the M29W128G chips as
reported by Richard Retanubun. This patch now introduces a weak default
function for the CFI reset command, still with both resets. This can
be overridden by a board specific version if necessary.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Richard Retanubun <RichardRetanubun@ruggedcom.com>