hw/block: m25p80: Add ISSI SPI flash support

hw/block: m25p80: Add ISSI SPI flash support

This adds the ISSI SPI flash support. The number of dummy cycles in
fast read, fast read dual output and fast read quad output commands
is currently using the default 8. Likewise, the same default value
is used for fast read dual/quad I/O command. Per the datasheet [1],
the number of dummy cycles is configurable, but this is not modeled
at present.

For flash whose size is larger than 16 MiB, the sequence of 3-byte
address along with EXTADD bit in the bank address register (BAR) is
not supported. We assume that guest software always uses op codes
with 4-byte address sequence. Fortunately, this is the case for both
U-Boot and Linux spi-nor drivers.

QPI (Quad Peripheral Interface) that supports 2-cycle instruction
has different default values for dummy cycles of fast read family
commands, and is unsupported at the time being.

[1] http://www.issi.com/WW/pdf/25LP-WP256.pdf

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210126060007.12904-2-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>