avb2.0: use block API in AVB ops

Use blk_dread()/blk_dwrite() in mmc_read()/mmc_write() AVB operation
implementations. This fixes compilation issues when CONFIG_BLK is
enabled.

Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Tested-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>