README: Simplify Xtensa toolchain summary description

Remove all the vendor-specific toolchain variants from what is supposed
to be a quick summary of the toolchains supported by Zephyr SDK.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

README.md

@@ -14,9 +14,7 @@ The toolchains for the following target architectures are supported:
 - RISC-V (32-bit and 64-bit; RV32I, RV32E, RV64I)
 - SPARC (32-bit and 64-bit; SPARC V8, SPARC V9)
 - x86 (32-bit and 64-bit)
-- Xtensa (sample_controller, intel_ace15_mtpm, intel_tgl_adsp,
-  nxp_imx_adsp, nxp_imx8m_adsp, nxp_imx8ulp_adsp, nxp_rt500_adsp,
-  espressif_esp32, espressif_esp32s2, espressif_esp32s3, mt8195_adsp)
+- Xtensa (sample_controller and other vendor-specific variants)
 
 The following host tools are available as part of the Zephyr SDK: