MIPS: Kconfig: add MIPS_GENERIC_KERNEL symbol

The MIPS_GENERIC symbol now won't select any other configuration option.
The MIPS_GENERIC_KERNEL will select all the options that the previous
MIPS_GENERIC option did select, and will select MIPS_GENERIC as well.

The whole point of this, is that it now becomes possible to compile a
kernel for a SoC supported by the arch/mips/generic/ code, without
making that kernel generic itself.

Signed-off-by: Paul Cercueil <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/Kconfig

···
 config MIPS_FIXUP_BIGPHYS_ADDR
 	bool
 
+config MIPS_GENERIC
+	bool
+
 menu "Machine selection"
 
 choice
 	prompt "System type"
-	default MIPS_GENERIC
+	default MIPS_GENERIC_KERNEL
 
-config MIPS_GENERIC
+config MIPS_GENERIC_KERNEL
 	bool "Generic board-agnostic MIPS kernel"
+	select MIPS_GENERIC
 	select BOOT_RAW
 	select BUILTIN_DTB
 	select CEVT_R4K

arch/mips/generic/Kconfig

···
 # SPDX-License-Identifier: GPL-2.0
-if MIPS_GENERIC
+if MIPS_GENERIC_KERNEL
 
 config LEGACY_BOARDS
 	bool