x86: ich-spi: Move opcode registers configuration to another routine

At present the ICH SPI opcode registers configuration is done in the
ich_spi_remove() routine, a little bit weird but that's how current.
Linux MTD driver works. This changes to move the opcode registers
configuration to a separate routine ich_spi_config_opcode() which
might be called by U-Boot itself as well.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>