mx31: make HSP clock for mx3fb driver available

This additionally updates mx31/generic.c by
- replacing __REG() macro accesses with readl() and writel()
- providing macros for PDR0 and PLL bit accesses

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>