powerpc: Rewrite FSL_BOOKE flush_cache_instruction() in C

Nothing prevents flush_cache_instruction() from being writen in C.

Do it to improve readability and maintainability.

This function is only use by low level callers, it is not
intended to be used by module. Don't export it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f989eff8296800c427622c0985384148404e4f0b.1597384512.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/misc_32.S

@@ -255,28 +255,6 @@ _ASM_NOKPROBE_SYMBOL(real_writeb)
 
 #endif /* CONFIG_40x */
 
-
-/*
- * Flush instruction cache.
- */
-#ifdef CONFIG_FSL_BOOKE
-_GLOBAL(flush_instruction_cache)
-#ifdef CONFIG_E200
-	mfspr   r3,SPRN_L1CSR0
-	ori     r3,r3,L1CSR0_CFI|L1CSR0_CLFC
-	/* msync; isync recommended here */
-	mtspr   SPRN_L1CSR0,r3
-	isync
-	blr
-#endif
-	mfspr	r3,SPRN_L1CSR1
-	ori	r3,r3,L1CSR1_ICFI|L1CSR1_ICLFR
-	mtspr	SPRN_L1CSR1,r3
-	isync
-	blr
-EXPORT_SYMBOL(flush_instruction_cache)
-#endif
-
 /*
  * Copy a whole page.  We use the dcbz instruction on the destination
  * to reduce memory traffic (it eliminates the unnecessary reads of