generic: 5.15: allow MV88E6xxx built-in when PTP support disabled

As described in commit 97c77fff28cf001399f33c7bc1ec6687ba18450b
MV88E6xxx driver (NET_DSA_MV88E6XXX) cannot be built-in when PTP
(PTP_1588_CLOCK) is a module in Linux 5.15. But actually it should be
allowed to be built-in when its PTP support (NET_DSA_MV88E6XXX_PTP) is
disabled.

This adds a patch to fix that.

Signed-off-by: Johnny S. Lee <foss@jsl.io>