As the PHYTEC phyCORE-i.MX91 [1] is just another variant of the existing
PHYTEC phyCORE-i.MX93 SoM but with i.MX91 SoC populated instead, add it
to the existing board-code "phycore_imx93", and rename that board to
"phycore_imx91_93" to reflect the dual SoCs support. While at it, also
rename and change common files accordingly. This way i.MX91 and i.MX93
SoC variants of the phyCORE SoM share most of the code and documentation
without duplication, while maintaining own device-tree and defconfigs
for each CPU variant.

Supported features:
 - 1GB LPDDR4 RAM
 - Debug UART
 - EEPROM
 - eMMC
 - Ethernet
 - SD-card
 - USB

Product page SoM:
[1] https://www.phytec.eu/en/produkte/system-on-modules/phycore-imx-91-93/

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>

The phyCORE-i.MX 93 is available in various variants. Relevant variant
options for the spl/u-boot are:
- with or without HS400 support for the eMMC
- with 1GB ram chip, or 2GB ram chip

The phyCORE's eeprom contains all information about the existing variant
options. Add evaluation of the eeprom data to the spl/u-boot to
enable/disable HS400 and to select the appropriate ram configuration at
startup.

Signed-off-by: Christoph Stoidner <c.stoidner@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Tested-by: Primoz Fiser <primoz.fiser@norik.com>