at91: Update meesc board to new SoC access

* convert meesc board to use c stucture SoC access
* change gpio access to at91_gpio syntax
* moved CONFIG_SYS_HZ below board and cpu defines (purely cosmetic)

Signed-off-by: Daniel Gorsulowski <Daniel.Gorsulowski@esd.eu>
Signed-off-by: Reinhard Meyer <u-boot@emk-elektronik.de>