m66, m53: Revise fdd-4msps-t2r2 init script

m66: reload FIR filter coefficients after enabling
m66: set sample rate after FIR filters
m53: readback FIR filters and sample rate after setting
m51: rename legacy HAL source
Refactor HAL to better match IIO driver

The original HAL was compatibile with a legacy driver provided by ADI, now unsupported and
not redistributable.  To better match the interfaces exposed by the new IIO driver and
ease writing applications and bindings using the library, this new HAL requires a device
number argument and offers primitives for reading and writing sysfs-style paths.

The legacy interface remains available, with device selection via a global variable
ad9361_legacy_dev, but is deprecated and should not be used for new applications.

Various HAL initialization functions previously done in the application layer have been
moved into the library with defaults matching the SDRDC platform.  These defaults can be
altered with functions such as ad9361_hal_linux_defaults_set_gpio() before calling the new
init function ad9361_hal_linux_init().
IIO debugfs skeleton generated
IIO sysfs skeleton generated
More sysfs API implementation

Implemented handling for variables and functions beyond single scalars:
- Enum handling code and current enum value/string mapping
- Fixed-point math for "hardwaregain" - for TX attenuation control
- FIR filter loading
- Clock path reporting
Command wrappers for new API calls

- Replaced old string-based sysfs/debugfs code in map-adi-iio.c with calls to new API
- Most commands use existing argument/result handling
- Typedefs changed from legacy library to stdint.h names
- Typos and mis-generated code corrected, vestigial files removed
Merged others' sdrdc-devel changes