MIPS: ath25: add board configuration detection

All boards based on AR5312/AR2315 SoC have a special structure located
at the end of flash. This structure contains board-specific data such as
Ethernet and Wireless MAC addresses. The flash is mapped to the memmory
at predefined location.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8243/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ath25/devices.h

@@ -7,8 +7,10 @@
 
 #define ATH25_IRQ_CPU_CLOCK	(MIPS_CPU_IRQ_BASE + 7)	/* C0_CAUSE: 0x8000 */
 
+extern struct ar231x_board_config ath25_board;
 extern void (*ath25_irq_dispatch)(void);
 
+int ath25_find_config(phys_addr_t offset, unsigned long size);
 void ath25_serial_setup(u32 mapbase, int irq, unsigned int uartclk);
 
 static inline bool is_ar2315(void)