ARC: [DeviceTree] Convert some Kconfig items to runtime values

* mem size now runtime configured (prev CONFIG_ARC_PLAT_SDRAM_SIZE)
* core cpu clk runtime configured (prev CONFIG_ARC_PLAT_CLK)

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@secretlab.ca>

arch/arc/include/asm/clk.h

@@ -17,4 +17,6 @@ static inline unsigned long arc_get_core_freq(void)
 	return core_freq;
 }
 
+extern int arc_set_core_freq(unsigned long);
+
 #endif