atmel_lcdfb: new alternate pixel clock formula

at91sam9g45 non ES lots have an alternate pixel clock calculation formula.
Introduce this one with condition on the cpu_is_xxxxx() macros.

Newer 9g45 SOC will not have good pixel clock calculation without this
fix.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>