MIPS: Simplify CONFIG_SYS_CPU values

Rather than having the values for CONFIG_SYS_CPU depend upon each
architecture revision, have them depend upon the more general
CONFIG_CPU_MIPS32 & CONFIG_CPU_MIPS64 which in turn depend upon the
architecture revisions.

This is done in preparation for adding MIPSr6 support, which would
otherwise need to introduce new cases here.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>

arch/mips/Kconfig

@@ -5,8 +5,8 @@ config SYS_ARCH
 	default "mips"
 
 config SYS_CPU
-	default "mips32" if CPU_MIPS32_R1 || CPU_MIPS32_R2
-	default "mips64" if CPU_MIPS64_R1 || CPU_MIPS64_R2
+	default "mips32" if CPU_MIPS32
+	default "mips64" if CPU_MIPS64
 
 choice
 	prompt "Target select"