cfi_flash: Introduce read and write accessors

Introduce flash_read{8,16,32,64) and flash_write{8,16,32,64} and use
them to access the flash memory. This makes it clearer when the flash
is actually being accessed; merely dereferencing a volatile pointer
looks just like any other kind of access.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>