board: MCR3000: cleanup config

Some config is redundant with Kconfig. Fix it.
Also remove unused configs
Move SDRAM_MAX_SIZE in the only place it is used

include/environment.h already defines CONFIG_ENV_SIZE
from CONFIG_ENV_SECT_SIZE and defines CONFIG_ENV_ADDR as
(CONFIG_SYS_FLASH_BASE + CONFIG_ENV_OFFSET)

remove BOOTARGS as bootargs is set by the different boot commands

Fix CONFIG_SYS_INIT_RAM_ADDR and CONFIG_SYS_INIT_RAM_SIZE to be in
line with CPM DPRAM organisation

Remove CONFIG_SYS_GBL_DATA_SIZE, CONFIG_SYS_GBL_DATA_OFFSET and
CONFIG_SYS_INIT_SP_OFFSET which are unused

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index f29465f..77c0333 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -32,6 +32,7 @@
 
 config MPC8xx
 	bool "MPC8xx"
+	select BOARD_EARLY_INIT_F
 	imply CMD_REGINFO
 
 endchoice