[beken-72xx] Add base BK7231 support config

docs/platform/beken-72xx/README.md

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+# Beken 72xx - notes
+
+There are many chip variations in this SoC family:
+
+- BK7231
+- BK7231T
+- BK7231N
+- BK7231S
+- BK7231U
+
+The "officially existing" ones are BK7231, BK7231N and BK7231U. These are supported by Beken SDKs, such as `bdk_freertos`, although `bk7231s_alios_sdk` also existed at some point.
+
+- BK7231N is substantially different than the other chips, so running T code on N (and vice versa) is not directly possible.
+- BK7231 does not have eFuse.
+- there are some references to U meaning USB support
+- T seems to be exclusive to Tuya boards (that would explain the name); in the T SDK from Tuya, `CFG_SOC_NAME` is set to `SOC_BK7231U`
+- T's bootloader greets with `BK7231S_1.0.5` on UART
+
+Regarding `bdk_freertos`:
+
+- `make` allows selecting for which MCU the code should be compiled
+- `make bk7231` doesn't compile at all
+- `make bk7231u` doesn't run on T with Tuya's bootloader (1.0.5), though it works just fine after replacing the bootloader with one of these included with `bdk_freertos` (1.0.8) (yes, even `bk7231n` bootloader)
+- after making a few changes to `driver/entry` and `driver/intc` (so it looks more like the code from T SDK) `bdk_freertos` runs just fine