ath79: ar724x: fix pll settings

ath79: ar724x: fix pll settings

Add the syscon compatible, otherwise used functions like
syscon_regmap_lookup_by_phandle() will return an error and setting the
ethernet pll data wont work at all.

Fix the pll register width. Writing to registers out of the range via
syscon isn't possible and returns an error. On ar7242 the last pll
register - Current Audio Modulation Logic Output - is at 0x1805003c.

Signed-off-by: Mathias Kresin <dev@kresin.me>