riscv: thead: Rename T-Head PBMT to MAE

T-Head's vendor extension to set page attributes has the name
MAE (memory attribute extension).
Let's rename it, so it is clear what this referes to.

Link: https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadmae.adoc


Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Link: https://lore.kernel.org/r/20240407213236.2121592-2-christoph.muellner@vrull.eu


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>