arch/mips: Make MIPS specific options depend on ARCH_MIPS

Also don't define the default as this result in spurious lines in the
.config.

TEST: The generated config.h remain exactly the same for all boards.

Change-Id: I7f35a5a9dcbc7b25b7806056e2b8e822fa94e428
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31312
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Martin Roth <martinroth@google.com>

src/arch/mips/Kconfig

@@ -16,12 +16,12 @@
 
 config ARCH_MIPS
 	bool
-	default n
+
+if ARCH_MIPS
 
 config ARCH_BOOTBLOCK_MIPS
 	bool
 	default n
-	select ARCH_MIPS
 	select BOOTBLOCK_CUSTOM
 	select C_ENVIRONMENT_BOOTBLOCK
 
@@ -36,3 +36,5 @@ config ARCH_ROMSTAGE_MIPS
 config ARCH_RAMSTAGE_MIPS
 	bool
 	default n
+
+endif # if ARCH_MIPS

src/soc/imgtec/pistachio/Kconfig

@@ -15,6 +15,7 @@
 #
 
 config CPU_IMGTEC_PISTACHIO
+	select ARCH_MIPS
 	select ARCH_BOOTBLOCK_MIPS
 	select ARCH_VERSTAGE_MIPS
 	select ARCH_ROMSTAGE_MIPS