risc-v/bl808: Configure MMU to cache User Text, Data and Heap

This PR configures the BL808 MMU to cache the the User Text, Data and Heap. We enable the T-Head MMU Flags for Shareable, Bufferable and Cacheable, as explained in the previous PR: apache#13199

This PR fixes the Slow Memory Access for NuttX Apps on Ox64 BL808 SBC: apache#12696. With this fix, Ox64 NuttX CoreMark jumps from 19 to 1,104. (Close to Buildroot Linux CoreMark)

Modified Files:

`arch/risc-v/Kconfig`: Enabled `ARCH_MMU_EXT_THEAD` for BL808 SoC.

arch/risc-v/Kconfig

@@ -295,6 +295,7 @@ config ARCH_CHIP_BL808
 	select ARCH_HAVE_MULTICPU
 	select ARCH_HAVE_MPU
 	select ARCH_MMU_TYPE_SV39
+	select ARCH_MMU_EXT_THEAD
 	select ARCH_HAVE_ADDRENV
 	select ARCH_NEED_ADDRENV_MAPPING
 	select ARCH_HAVE_S_MODE