[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

[zircon][rtc] Fixes and some tests for rtclib

The new fallback driver needs to return sizeof(struct)
rather than ZX_OK. I was misled by the DDKTL.

Doing some cleanup and noticed no tests. Also made it
so there are less magic numbers.

TEST= unittest included. Also manual test on Pixelbook.

Change-Id: I5531d3888bae601179b4fcdd0949a3fdcdc6768f

[rtc] share some common rtc code

- hoist some general-purpose code out of the intel-rtc driver
- call clock adjustment system call from vim2 driver

Test: verified ZX_CLOCK_UTC behaves correctly on a Vim2
Change-Id: I12577779826d99e392e70f29998647a2f111395f

[rtc] fix ioctl_rtc_set return value to match intel driver

Test: boot logging from network_time in garnet
Change-Id: I72c24bef3d96c74bd795dd45e1cd04320dcb0cb7

[dev][rtc] basic implementation for VIM RTC

- ZX-1985

Test: 'clock' utility works as expected
Change-Id: I1646342bf8052ed275303a99affd07e0fde8ffcb