mtd: nand: lpc32xx_slc: improve SLCTAC_*() macro definitions

No functional change, move bitfield calculations to macro
definitions with added clock rate argument, which are in turn defined
by new common SLCTAC_CLOCKS(c, n, s) macro definition.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>