MIPS: Kconfig: Fix typo for the r2-to-r6 emulator kernel parameter

MIPS: Kconfig: Fix typo for the r2-to-r6 emulator kernel parameter

Commit b0a668fb2038 ("MIPS: kernel: mips-r2-to-r6-emul: Add R2 emulator
for MIPS R6") added the mips r2-to-r6 emulator so an R2 userland can be
executed on R6 kernels. This needed both build time and runtime support.
The runtime support needed the "mipsr2emu" kernel parameter instead of
the "mipsr2emul" listed in the Kconfig help message.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Fixes: b0a668fb2038 ("MIPS: kernel: mips-r2-to-r6-emul: Add R2 emulator for MIPS R6")
Cc: linux-mips@linux-mips.org
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9504/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>