aspeed/smc: Add extra controls to request DMA

The AST2600 SPI controllers have a set of bits to request/grant DMA
access. Add a new SMC feature for these controllers and use it to
check access to the DMA registers.

Cc: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20210407171637.777743-16-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>