mmc: mtk-sd: fix sclk cycles shift value

It turns out that the sclk cycles used by mt7620/mt7628 is the same as
other chips (20 bits, 1048576), not 65536.

This patch removes sclk_cycle_shift from dev_comp, and uses a macro with
a value 20 instead.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>