MIPS: Select R3k-style TLB in Kconfig

Currently areas where we need to determine whether the TLB is R3k-style
need to check for either of CONFIG_CPU_R3000 || CONFIG_CPU_TX39XX.

Introduce a new CONFIG_CPU_R3K_TLB & select it from both of the above,
allowing us to simplify checks for R3k-style TLBs by only checking for
this new Kconfig option.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: linux-mips@vger.kernel.org