board: MCR3000: Use lowercase filenames

Rename MCR3000.* to mcr3000.* to be more in line with
other boards.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
diff --git a/arch/powerpc/cpu/mpc8xx/Kconfig b/arch/powerpc/cpu/mpc8xx/Kconfig
index d630711..65293ae 100644
--- a/arch/powerpc/cpu/mpc8xx/Kconfig
+++ b/arch/powerpc/cpu/mpc8xx/Kconfig
@@ -84,6 +84,6 @@
 	help
 	  Debug Event Register (37-47)
 
-source "board/cssi/MCR3000/Kconfig"
+source "board/cssi/mcr3000/Kconfig"
 
 endmenu