Added support for ISSI IS25LP016D SPI flash and EXTERNAL_FLASH_DEVICES on variant.h

[nekuneko]

I would like to add Arduino support for this tiny 2MiB USON8 2x3mm flash memory.
Nowadays this flash is supported for CircuitPython on this repository:

https://github.com/adafruit/nvm.toml/tree/main/flash/issi

Nevertheless I feel some inconsistency regarding the start_up_time_us, quad_enable_bit_mask and single_status_byte parameters. Reading the datasheet I think the stated ones here are the correct ones but I'm not completely sure.

Also It would be really nice to specify at documentation in which unit total_size is represented, finally I managed to find out is represented in bytes. It's a bit mess for newcomers because manufacturers uses the notation Mb (Megabit) to refer Mib (Mebibits). In this case, on datasheet, the memory is stated to be a 16Mb (Megabit) but in fact it has 512 sectos of "4KB", here again the manufacturer uses KB (Kilobytes) for meaning 4KiB (Kibibytes), this is 4096 bytes, so finally the memory has 16Mib (Mebibits) or which is the same, 2MiB (Mebibytes), and this is 2097152 bytes. (See Table 5.1 Block/Sector Addresses).

When executing the flash_info example sketch the output given is;

Adafruit Serial Flash Info example
JEDEC ID: 0x9D6015
Flash size: 2048 KB

Here the flash size is given in KB (Kilobytes) instead of KiB (Kibibytes), so the confusion continues ^^".

Thank you very much.

PS: Why did all checking build jobs failed? xd

[hathach]

thank you for your PR. please update change per review. To pass ci clang (standard code formatter). You need to install clang-format (sudo apt install clang-format) and run following commands:

clang-format -i src/*