[ddk][i2c] Refactor the I2C protocol

This change is similar to "[gpio] Refactor the GPIO protocol" but for
I2C channels: https://fuchsia-review.googlesource.com/c/zircon/+/199400

After this change an instance of the ZX_PROTOCOL_I2C protocol
now represents a single I2C channel. The protocol remains the same,
except the "uint32_t index" has been removed from all the protocol functions.

For devices that have only one I2C channel assigned to them, the
driver can simply call device_get_protocol() to access the I2C protocol for the
channel. To support multiple channel, a new API in the platform bus called
pdev_get_protocol() allows accessing the I2C channels protocols by index.

ZX-2621 #comment Refactor the I2C protocol removing index.

Test: Boot and I2C transactions work on astro and NXP IMX8

Change-Id: I55d08aa3d82024f6977646425fd3d75a378f2740

[ddk][i2c] Add arbitrary number of RWs support to I2C protocol

Augment the I2C protocol transact function to allow more than 2
write and/or read operations in one transaction. Rename the old
transact to write_read.

Test: Astro light sensor (uses I2C based interface) readings work

Change-Id: I3c83025b7ca697551638ed9824cb7547e37afc4b

[led2472g] Re-enable driver

When i2c_transact fails, it prints 'i2c: error on bus', so when we get
the first i2c_error, we quit the display thread.

Change-Id: If08c168a4050dd5875fc3a44038a550da5362bb3

[drivers] Add driver for RPi Sense HAT LED2472G

Change-Id: I78a0ab042733fe70a4bebf73dbe2bbf73ae3bbef