hw/sd: ssi-sd: Add a state representing Nac

Per the "Physical Layer Specification Version 8.00" chapter 7.5.2,
"Data Read", there is a minimum 8 clock cycles (Nac) after the card
response and before data block shows up on the data out line. This
applies to both single and multiple block read operations.

Current implementation of single block read already satisfies the
timing requirement as in the RESPONSE state after all responses are
transferred the state remains unchanged. In the next 8 clock cycles
it jumps to DATA_START state if data is ready.

However we need an explicit state when expanding our support to
multiple block read in the future. Let's add a new state PREP_DATA
explicitly in the ssi-sd state machine to represent Nac.

Note we don't change the single block read state machine to let it
jump from RESPONSE state to DATA_START state as that effectively
generates a 16 clock cycles Nac, which might not be safe. As the
spec says the maximum Nac shall be calculated from several fields
encoded in the CSD register, we don't want to bother updating CSD
to ensure our Nac is within range to complicate things.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210123104016.17485-9-bmeng.cn@gmail.com>
[PMD: Change VMState version id 4 -> 5]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

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